fix(teams): close remaining cleanup gaps for worktrees and session dirs

[tmustier]

Summary

Closes remaining cleanup gaps for worktrees and session directories (SYM-41 / #9).

PRs #14 and #30 added the core cleanup infrastructure (cleanupWorktrees, gcStaleTeamDirs, session_shutdown handler). This PR closes the remaining gaps:

Changes

1. Standalone git worktree prune on startup (worktree.ts)

• New pruneStaleWorktreeRefs() function — runs git worktree prune on the working repo
• Called in session_start to clean up dangling .git/worktrees/ entries from sessions whose team directories were already deleted (crash, manual rm, partial cleanup)
• Lightweight and safe to run every startup; only removes bookkeeping for worktree dirs that no longer exist

2. Pass repoCwd to startup GC (leader.ts)

• gcStaleTeamDirs now receives ctx.cwd so cleanupWorktrees can find the repo root even when worktree directories are gone from disk

3. Smarter session_shutdown cleanup (leader.ts)

• Replaces teamDirHasAnyTasks with teamDirHasActiveTasks — checks for pending/in-progress tasks only
• Team dirs with only completed tasks are now cleaned up at session end instead of waiting 24h for startup GC
• Completed tasks have already been reported back to the leader and serve no ongoing purpose

4. Remove hadTeammates guard (leader.ts)

• session_shutdown now attempts dir cleanup regardless of whether RPC teammates were active
• Safety checks (no live attach claim, no online workers, no active tasks) are sufficient guards
• Previously, any session that used RPC teammates left its dir for 24h even when all work was done

5. Pass repoCwd to cleanupTeamDir in shutdown (leader.ts)

• Ensures proper worktree metadata cleanup even when the team dir's worktree subdirs are being removed

Testing

• 8 new integration tests in integration-cleanup-test.mts:
  • pruneStaleWorktreeRefs cleans orphaned .git/worktrees/ metadata
  • pruneStaleWorktreeRefs handles non-git directories gracefully
  • gcStaleTeamDirs removes dirs with only completed tasks
• All 325 smoke tests pass
• All 56 integration-cleanup tests pass (48 existing + 8 new)
• Lint + typecheck clean (no new errors)

Resolves #9

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Treat unreadable task JSON as active during shutdown cleanup

teamDirHasActiveTasks now skips task files that fail readFile/JSON.parse, so a team with malformed or partially written task JSON can be misclassified as having no active tasks; in session_shutdown that allows cleanupTeamDir(...) to delete the entire team directory and lose unresolved task data. The previous teamDirHasAnyTasks logic was fail-closed for this case, so this change introduces a new data-loss path when task files are corrupted or in an unexpected format.

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