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>

src/heartbeat.py

@@ -316,6 +316,10 @@ def _check_kb_index() -> str | None:
 
     newer = 0
     for md in kb_dir.rglob("*.md"):
+        # Skip generated nav files — they're written by the reindex itself, so
+        # comparing them against index.json mtime would cause perpetual reindex.
+        if md.name == "index.md":
+            continue
         if md.stat().st_mtime > index_mtime:
             newer += 1