feat: Read My Mind phase 2, the predictor and the brain button

The feature as pitched in docs/readmymind-plan.md: pressing the header
brain button predicts the prompt you were about to type, from the case's
intent profile plus everything the session already knows.

Backend:
- readmymind-context.ts: pure budgeted context assembler (9 ranked
  sources: pending approval dialog, user goals, last assistant turn tail,
  recent prompts, tool activity, git workspace signals, away context,
  sibling sessions, rethink state; 30 KB budget, whole-section drop from
  the bottom of the ranking, trust tiers stated in the prompt)
- readmymind-collectors.ts: transcript tail reader (the live watcher
  keeps only a 500-char snippet) and git signal collection (execFile,
  2s timeout, skipped for remote-SSH cases)
- readmymind-predictor.ts: one-shot claude -p in a throwaway tmux
  session, opus by default (readMyMindModel setting), strict JSON
  contract with 1-3 suggestions (continue / verify / redirect), newline
  stripping, 90s timeout; mutable singleton so route tests can stub it
- POST /api/sessions/:id/readmymind: claude-mode only (400), one
  prediction in flight per session (409 CONFLICT), rethink body
  { steer, rejected }; ownership via findSessionOrFail

Frontend:
- readmymind-ui.js (loadorder 11.3): header brain button, marker-hidden
  until readMyMindEnabled is ON, desktop only (phone key is phase 3);
  modal with editable suggestion + rationale and Send / Insert /
  Rethink / Dismiss; suggestion text rendered via value/textContent only
  and nothing ever auto-sends
- App Settings -> Panels checkbox for readMyMindEnabled; en + zh-CN
  strings

Verified end to end against a live isolated instance: transcript
capture, a real opus prediction grounded in the stated goals, rethink
steering, the 409, and the browser modal incl. Insert leaving the text
unsubmitted on the composer. 41 new unit/route tests; full test:ci
sweep green (4680 tests).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/readmymind-predictor.ts

@@ -0,0 +1,246 @@
+/**
+ * @fileoverview Read My Mind predictor: one-shot `claude -p` over the
+ * assembled prediction context (docs/readmymind-plan.md).
+ *
+ * Reuses the AiCheckerBase spawn mechanics (prompt file to dodge E2BIG, a
+ * throwaway detached tmux session, done-marker polling, timeout, shell-safety
+ * validation) but stays standalone: the base class is verdict-shaped
+ * (positive/negative/cooldown) and prediction is freeform JSON, so subclassing
+ * would abuse `reasoning` as a payload.
+ *
+ * The predictor is deliberately dumb, text in / JSON out; all intelligence
+ * about WHAT to include lives in the testable assembler
+ * (`readmymind-context.ts`). Output parsing (`parsePredictionOutput`) is pure
+ * and strict: garbage output is a clean error, never a half-suggestion, and
+ * suggestion prompts are collapsed to single lines server-side (multi-line
+ * breaks Ink).
+ *
+ * Exported as a mutable singleton (`readMyMindPredictor`) so route tests can
+ * stub `predict` without spawning anything.
+ */
+
+import { execSync, spawn as childSpawn } from 'node:child_process';
+import { existsSync, readFileSync, unlinkSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { z } from 'zod';
+import { isValidModelName, isValidMuxName } from './ai-checker-base.js';
+import { getAugmentedPath } from './utils/index.js';
+import { getErrorMessage } from './types.js';
+
+// ========== Contract ==========
+
+export type SuggestionKind = 'continue' | 'verify' | 'redirect';
+
+export interface ReadMyMindSuggestion {
+  /** The proposed next prompt: single line, bounded. */
+  prompt: string;
+  /** One-sentence rationale. */
+  why: string;
+  kind: SuggestionKind;
+}
+
+export interface PredictionResult {
+  suggestions: ReadMyMindSuggestion[];
+  durationMs: number;
+}
+
+/** Opus headroom over a ~30 KB prompt (decided in the design doc). */
+export const READMYMIND_TIMEOUT_MS = 90_000;
+
+const MAX_SUGGESTION_CHARS = 1_000;
+const MAX_WHY_CHARS = 300;
+const DONE_MARKER = '__RMM_DONE__';
+const POLL_INTERVAL_MS = 500;
+
+/** Lenient on extra keys (zod strips unknowns), strict on shape. */
+const SuggestionsSchema = z.object({
+  suggestions: z
+    .array(
+      z.object({
+        prompt: z.string(),
+        why: z.string().optional(),
+        kind: z.enum(['continue', 'verify', 'redirect']),
+      })
+    )
+    .min(1)
+    .max(3),
+});
+
+/** Collapse to one line: embedded newlines break Ink's composer. */
+function singleLine(text: string): string {
+  return text.replace(/\s*[\r\n]+\s*/g, ' ').trim();
+}
+
+/**
+ * Parse the model's raw output into validated suggestions. Strict by design:
+ * anything that does not contain the JSON contract is an Error, never a
+ * half-suggestion. Tolerates fenced/prosed wrapping by extracting the
+ * outermost object literal before parsing.
+ */
+export function parsePredictionOutput(raw: string): ReadMyMindSuggestion[] {
+  const start = raw.indexOf('{');
+  const end = raw.lastIndexOf('}');
+  if (start === -1 || end <= start) {
+    throw new Error('Predictor returned no JSON object');
+  }
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw.slice(start, end + 1));
+  } catch {
+    throw new Error('Predictor returned malformed JSON');
+  }
+
+  const result = SuggestionsSchema.safeParse(parsed);
+  if (!result.success) {
+    throw new Error('Predictor output did not match the suggestions contract');
+  }
+
+  const suggestions = result.data.suggestions
+    .map((s) => ({
+      prompt: singleLine(s.prompt).slice(0, MAX_SUGGESTION_CHARS),
+      why: singleLine(s.why ?? '').slice(0, MAX_WHY_CHARS),
+      kind: s.kind,
+    }))
+    .filter((s) => s.prompt.length > 0);
+
+  if (suggestions.length === 0) {
+    throw new Error('Predictor returned only empty suggestions');
+  }
+  return suggestions;
+}
+
+// ========== Spawn/poll runner ==========
+
+export interface PredictOptions {
+  /** Codeman session id; only its first 8 chars name the throwaway tmux session. */
+  sessionId: string;
+  /** The assembled context prompt (readmymind-context.ts). */
+  prompt: string;
+  /** Model name; shell-validated before use. */
+  model: string;
+  timeoutMs?: number;
+}
+
+async function runPrediction(options: PredictOptions): Promise<PredictionResult> {
+  const { sessionId, prompt, model } = options;
+  const timeoutMs = options.timeoutMs ?? READMYMIND_TIMEOUT_MS;
+
+  if (!isValidModelName(model)) {
+    throw new Error(`Invalid model name: ${String(model).substring(0, 50)}`);
+  }
+
+  const shortId = sessionId.replace(/[^a-zA-Z0-9_-]/g, '').slice(0, 8) || 'rmm';
+  const timestamp = Date.now();
+  const outFile = join(tmpdir(), `codeman-rmm-${shortId}-${timestamp}.txt`);
+  const stderrFile = join(tmpdir(), `codeman-rmm-stderr-${shortId}-${timestamp}.txt`);
+  const promptFile = join(tmpdir(), `codeman-rmm-prompt-${shortId}-${timestamp}.txt`);
+  const muxName = `codeman-rmm-${shortId}`;
+  if (!isValidMuxName(muxName)) {
+    throw new Error(`Invalid mux name generated: ${muxName.substring(0, 50)}`);
+  }
+
+  writeFileSync(outFile, '');
+  writeFileSync(stderrFile, '');
+  // Prompt via file + stdin: ~30 KB exceeds argv comfort (E2BIG).
+  writeFileSync(promptFile, prompt, { mode: 0o600 });
+
+  const modelArg = `--model "${model.replace(/"/g, '\\"')}"`;
+  const claudeCmd = `cat "${promptFile}" | claude -p ${modelArg} --output-format text`;
+  const fullCmd = `export PATH="${getAugmentedPath()}"; ${claudeCmd} > "${outFile}" 2> "${stderrFile}"; echo "${DONE_MARKER}" >> "${outFile}"; rm -f "${promptFile}"`;
+
+  const startTime = Date.now();
+  let pollTimer: NodeJS.Timeout | null = null;
+  let timeoutTimer: NodeJS.Timeout | null = null;
+
+  const cleanup = (): void => {
+    if (pollTimer) clearInterval(pollTimer);
+    if (timeoutTimer) clearTimeout(timeoutTimer);
+    pollTimer = null;
+    timeoutTimer = null;
+    try {
+      execSync(`tmux kill-session -t "${muxName}" 2>/dev/null`, { timeout: 2000 });
+    } catch {
+      // Session already gone.
+    }
+    for (const file of [outFile, stderrFile, promptFile]) {
+      try {
+        if (existsSync(file)) unlinkSync(file);
+      } catch {
+        // Best-effort cleanup.
+      }
+    }
+  };
+
+  try {
+    try {
+      execSync(`tmux kill-session -t "${muxName}" 2>/dev/null`, { timeout: 3000 });
+    } catch {
+      // No leftover session: fine.
+    }
+    const muxProcess = childSpawn('tmux', ['new-session', '-d', '-s', muxName, 'bash', '-c', fullCmd], {
+      detached: true,
+      stdio: 'ignore',
+    });
+    muxProcess.unref();
+  } catch (err) {
+    cleanup();
+    throw new Error(`Failed to spawn prediction tmux session: ${getErrorMessage(err)}`);
+  }
+
+  return new Promise<PredictionResult>((resolve, reject) => {
+    let settled = false;
+
+    pollTimer = setInterval(() => {
+      if (settled) return;
+      try {
+        if (!existsSync(outFile)) return;
+        const content = readFileSync(outFile, 'utf-8');
+        if (!content.includes(DONE_MARKER)) return;
+        settled = true;
+        const durationMs = Date.now() - startTime;
+        const output = content.replace(DONE_MARKER, '').trim();
+        if (!output) {
+          const stderr = readStderr(stderrFile);
+          cleanup();
+          reject(new Error(`Predictor produced no output${stderr ? `: ${stderr}` : ''}`));
+          return;
+        }
+        try {
+          const suggestions = parsePredictionOutput(output);
+          cleanup();
+          resolve({ suggestions, durationMs });
+        } catch (err) {
+          cleanup();
+          reject(err instanceof Error ? err : new Error(getErrorMessage(err)));
+        }
+      } catch {
+        // Output file mid-write or already removed: keep polling.
+      }
+    }, POLL_INTERVAL_MS);
+
+    timeoutTimer = setTimeout(() => {
+      if (settled) return;
+      settled = true;
+      cleanup();
+      reject(new Error(`Prediction timed out after ${timeoutMs}ms`));
+    }, timeoutMs);
+  });
+}
+
+function readStderr(stderrFile: string): string {
+  try {
+    return existsSync(stderrFile) ? readFileSync(stderrFile, 'utf-8').trim().substring(0, 200) : '';
+  } catch {
+    return '';
+  }
+}
+
+/**
+ * Mutable singleton: routes call `readMyMindPredictor.predict(...)`; tests
+ * stub the property (`vi.spyOn(readMyMindPredictor, 'predict')`).
+ */
+export const readMyMindPredictor = {
+  predict: runPrediction,
+};