fix(session): resume the conversation when respawning a dead pane

A CLI that launches with `--session-id <id>` refuses an id that is already in
use (claude: `Error: Session ID ... is already in use.`), and every session
whose agent has been prompted owns a transcript under that id. The dead-pane
respawn in `_setupOrAttachMuxSession()` passed the bare launch line, so
recovering such a session relaunched a CLI that died on startup, the pane went
dead again at once, and the conversation was stranded behind a tab that looked
merely idle.

`restartCli()` has pinned a resume id against this since the custom-model work,
and its comment states the assumption that made the other path look safe:
"Unlike the dead-pane respawn, this one kills a WORKING pane whose conversation
already has a transcript". A pane whose agent exited has a transcript too.

Both relaunch paths now build options through
`_buildRespawnPaneOptionsWithResumePin()`, and so does the create-path fallback
after a failed respawn, which otherwise met the same refusal that made it the
fallback. Four gates guard the pin, each standing for a way of resuming the
WRONG conversation or of making a working relaunch fail.

A remote or docker session is never pinned. Unlike `restartCli()`, whose route
refuses both, the dead-pane respawn is reached by every session shape. Their
pane commands already render a self-healing `--session-id || --resume`, and
both flip to resume-first once the resume id differs; the conversation lives on
the far side, so a local id resolves to nothing there and the `--session-id`
fallback then collides with the transcript the far side does hold.

The id comes from the conversation CHAIN rather than `_claudeSessionId`, which
also holds history-correlated guesses keyed on the working directory.
`_recordClaudeSessionInChain()` refuses those so they cannot "write a foreign
conversation into this pane's permanent record", and launching from one is
worse than the display bug that rule prevents. The chain tail also outranks the
launch seed, which is written once at construction and never moves off a
`/clear`.

A pin no transcript backs is dropped, because the fallback branch keeps
`--session-id <this.id>` and would collide. A synthetic `restored-<fragment>`
id from socket discovery is dropped too, and logged: it fails claude's `uuid`
token pattern, so the renderer would emit the unpinned command while the caller
believed otherwise.

Tests cover each gate and the rendered command. Four of them fail against the
unfixed source; the remote and docker ones were separately checked against a
build with only that guard removed, since they pass on master for the wrong
reason.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/utils/claude-transcript.ts

@@ -0,0 +1,57 @@
+/**
+ * @fileoverview Does a Claude conversation transcript exist on this host?
+ *
+ * Claude writes one `<conversation-id>.jsonl` per conversation under
+ * `<config dir>/projects/<mangled cwd>/`. Two launch decisions turn on whether
+ * such a file exists: `--resume <id>` needs one, and `--session-id <id>` is
+ * REFUSED when one exists (`Error: Session ID ... is already in use.`).
+ *
+ * The project directory name is derived from the working directory, and a case
+ * that has been moved or renamed leaves its transcript under the OLD name, so
+ * the search is across every project directory rather than the one that matches
+ * the pane's cwd today.
+ *
+ * ⚠️ Existence is the whole question here, with no size floor. The create route
+ * additionally requires ~4 KB before it will resume, which is a "is this
+ * conversation worth resuming" judgement; for a relaunch the question is the
+ * opposite one — a one-line transcript still makes `--session-id` collide.
+ *
+ * @dependencies none
+ * @consumedby session (relaunch resume pinning)
+ *
+ * @module utils/claude-transcript
+ */
+
+import { readdir, stat } from 'node:fs/promises';
+import { homedir } from 'node:os';
+import { join } from 'node:path';
+
+/** `<config dir>/projects`, honouring a session's relocated `CLAUDE_CONFIG_DIR`. */
+export function claudeProjectsDir(configDir?: string): string {
+  return join(configDir || join(homedir(), '.claude'), 'projects');
+}
+
+/**
+ * True when a transcript for `conversationId` exists under any project
+ * directory. Returns false for a missing projects dir or an unreadable one:
+ * the caller's fallback is to skip the resume, which is the safe direction.
+ */
+export async function claudeTranscriptExists(conversationId: string, configDir?: string): Promise<boolean> {
+  if (!conversationId) return false;
+  const projectsDir = claudeProjectsDir(configDir);
+  let projectDirs: string[];
+  try {
+    projectDirs = await readdir(projectsDir);
+  } catch {
+    return false;
+  }
+  for (const projectDir of projectDirs) {
+    try {
+      await stat(join(projectsDir, projectDir, `${conversationId}.jsonl`));
+      return true;
+    } catch {
+      // Not in this project directory; keep looking.
+    }
+  }
+  return false;
+}