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>

docs/api-reference.md

@@ -454,8 +454,20 @@ user guide: [`readmymind.md`](readmymind.md).
   `400 INVALID_INPUT` on over-long or unknown fields.
 - `DELETE /api/v1/sessions/:id/intent` -> `{ deleted: boolean }` forgets the
   case's profile entirely.
+- `POST /api/v1/sessions/:id/readmymind` predicts the user's next prompt:
+  a one-shot model call over the intent profile plus live session signals
+  (pending approval dialog, transcript tail, git state, run-summary events,
+  sibling sessions). Body is optional; the rethink flow passes
+  `{ steer?, rejected? }` (strict schema: `steer` <= 2000 chars, `rejected`
+  up to 10 strings <= 1000 chars). Answers
+  `{ suggestions: { prompt, why, kind }[], durationMs }` with 1-3 suggestions
+  (`kind`: `continue` | `verify` | `redirect`; prompts are single-line).
+  Claude-mode sessions only (`400 INVALID_INPUT` otherwise); one prediction in
+  flight per session (`409 CONFLICT`); predictor failures answer
+  `502 OPERATION_FAILED`. Takes 5-90 s and costs real tokens. Suggestions are
+  only ever returned, never sent: submitting one is the caller's explicit act.
 
-All three enforce session ownership in multi-user mode; a foreign session id
+All four enforce session ownership in multi-user mode; a foreign session id
 answers `404 NOT_FOUND` (no existence leak), and profiles of two owners of the
 same directory are distinct by construction.