Add Thread Organizer plugin

[brsbl]

Summary

• Add a headless Thread Organizer plugin that classifies newly created user threads into the existing bb, Extensions, Design, or Writing sections.
• Evaluate at creation, retry when the initial prompt becomes available, then revisit after completed turns 1, 5, 15, 25, and every ten turns thereafter.
• Repair only titles that remain missing after a completed turn; native titles and all manual titles or moves permanently win.
• Start in observe mode, never create sections, never backfill existing threads, and keep QA, pinning, and archiving manual.

Safety

• Deterministic local rules with confidence and runner-up-margin gates; no hidden classifier thread or custom UI.
• Stricter two-evaluation confirmation before moving a plugin-managed thread.
• Fresh reads before writes and fail-closed behavior for missing or duplicate section aliases.
• Excludes hidden workers, child/fork/side-chat threads, plugin-originated threads, archived/deleted threads, and terminal error states.
• Persists only per-thread cadence, override locks, and last plugin-authored values; archived/deleted state is removed.

Validation

• npm run check
• 54 focused tests, including 11 representative historical classifications, confirmed two-milestone cross-section moves, active-phase persistence, and mixed-intent abstention
• Repository hygiene, all workspace typechecks/builds/tests, clean-install scaffold smoke, built tests, and packed artifact validation pass
• Federated review found one P1 in periodic reorganization; live QA exposed its remaining lifecycle reset path, and both fixes passed bounded closure review with no remaining P0/P1 findings
• Isolated bb dev instance on current origin/main:
  • A real visible personal-workspace thread was initially assigned to the existing Writing section.
  • After its purpose changed, it stayed in Writing at the first scheduled milestone and moved to the existing Extensions section only at the second.
  • The native title remained unchanged throughout.
  • Observe mode separately verified creation, activation, completed-turn timing, initial-prompt availability, and no mutations.
• Live path install on bb 0.33.0 remains in observe mode; no existing user threads were modified