feat(memory): hybrid retrieval — navigation index.md + RAG hardening

Expose a navigation layer to the agent and harden RAG, after analyzing the
OKF note and testing on the real KB.

- memory_search.search(): dedupe best-chunk-per-file (a relevant note can no
  longer be buried by another file's chunks) + keyword fallback tagged
  degraded:True when Ollama is unreachable (no more hard crash).
- update_notes_index.py: emit per-folder index.md + root router; prune empty
  folders; fix latent subcategory->project bug.
- Exclude generated index.md from RAG rglob (reindex/incremental) + indexer
  scans + heartbeat freshness check (prevents self-pollution / reindex thrash).
- CLAUDE.md: reframe memory as hybrid (navigation first, RAG for fuzzy recall).
- Delete stale orphan kb/youtube/index.json; correct the OKF source note.
- Tests: dedup, keyword fallback, index.md exclusion. Plan + review in docs/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

memory/kb/retete/index.md

@@ -0,0 +1,6 @@
+# Index — retete/
+
+> 1 note. Citește acest index întâi; deschide doar fișierele relevante.
+
+- **[Ciorbă de Burtă Falsă cu Pui și Ciuperci Pleurotus](2026-01-30_ciorba-burta-falsa-cu-pui.md)**  `@health #ciorba #reteta #pleurotus #pui`
+  - Se poate face și de post: fără carne, cu lapte vegetal în loc de smântână