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/exercitii/index.md

@@ -0,0 +1,12 @@
+# Index — exercitii/
+
+> 4 note. Citește acest index întâi; deschide doar fișierele relevante.
+
+- **[Diagnostic Platou Financiar - Chestionar](diagnostic-platou-financiar.md)**
+  **Sursă:** Friday Spark #183 - 8 Blocaje Financiare Invizibile
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+  **Actualizat:** 2026-02-06
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+- **[Exercițiu: Reframe Credințe Limitatoare (NLP)](reframe-credinte-limitatoare.md)**  `@work @growth #nlp #credinte #reframe`
+  **Next step:** Când apare gândul "nu sunt destul de bun", deschide fișierul și citește Reframe #1