docs: move 6 root .md files into docs/progress and docs/_archive

- Move TODO.md, TODAY_SUMMARY.md, SESSION_RECONSTITUTION.md,
  BATCH_A_COMPLETION_SUMMARY.md to docs/progress/
- Move PR_DESCRIPTION.md, MERGE_READY_SUMMARY.md to docs/_archive/
- Update docs/00-INDEX.md with new progress and archive entries
- Note moves in docs/_archive/2025-12-16-consolidation/CONSOLIDATION_SOURCE_MAP.md

Root keeps only README, CHANGELOG, CONTRIBUTING, SECURITY, BUILDING,
ARCHITECTURE, API, USAGE.

docs/00-INDEX.md

@@ -35,6 +35,10 @@ These files define the current truth about project state, decisions, and verific
 - **[05-CHATGPT-FEEDBACK-PACKAGE.md](./progress/05-CHATGPT-FEEDBACK-PACKAGE.md)** — AI collaboration package
 - **[P2-DESIGN.md](./progress/P2-DESIGN.md)** — P2 scope, invariants, and acceptance criteria (design-only)
 - **[P2.1-REFACTORING-COMPLETE.md](./progress/P2.1-REFACTORING-COMPLETE.md)** — P2.1 native plugin refactoring complete summary (Android + iOS)
+- **[TODO.md](./progress/TODO.md)** — Project TODO and improvement tracking
+- **[TODAY_SUMMARY.md](./progress/TODAY_SUMMARY.md)** — Dated work summaries
+- **[SESSION_RECONSTITUTION.md](./progress/SESSION_RECONSTITUTION.md)** — P2.1 Batch A session reconstitution
+- **[BATCH_A_COMPLETION_SUMMARY.md](./progress/BATCH_A_COMPLETION_SUMMARY.md)** — P2.1 Batch A completion summary
 
 ---
 
@@ -52,6 +56,7 @@ These files define the current truth about project state, decisions, and verific
 ## Archive & Reference-only
 
 - **`docs/_archive/`** — Historical artifacts, preserved for audit trail (not part of active doc surface)
+  - `docs/_archive/PR_DESCRIPTION.md`, `MERGE_READY_SUMMARY.md` — One-off PR/merge artifacts (2025-10)
   - `docs/_archive/2025-legacy-doc/` — Legacy documentation from 2025
     - [IMPLEMENTATION_CHECKLIST_LEGACY.md](./_archive/2025-legacy-doc/IMPLEMENTATION_CHECKLIST_LEGACY.md) — iOS Phase 1 checklist (historical)
   - `docs/_archive/2025-12-16-consolidation/` — 2025-12-16 consolidation artifacts (audit trail)