feat(retrieval): reranker, type-weighted graph, MMR, importance, passage-window (v3.9.0)

[Vahlame]

Summary

Makes the kit's retrieval more accurate with five opt-in, off-by-default levers — each measured, with the default path byte-identical (deterministic retrieval-bench unchanged: recall@5 1.000 / MRR 0.984 / hit@1 0.969 / nDCG 0.988 / MAP 0.984) — plus real-defect hardening across Go/Node/CI.

Retrieval accuracy (ADR-0026/0027/0028)

• Cross-encoder reranker behind the new [rerank] extra (fastembed TextCrossEncoder, lazy, durable cache, fail-safe fallback). CLI --rerank/--rerank-model, env OBSIDIAN_MEMORY_RERANK, and vault_hybrid_search(rerank: true). Default model is multilingual jina-reranker-v2. Off by default.
• Type-weighted graph recall (graphTyped) — verb-weighted typed relations (supersedes/implements > relates_to).
• Importance / in-degree (importance) — bounded hub boost; completes the relevance×recency×importance triad.
• MMR diversification (mmr) + passage-window expansion (passageWindow, ranking-neutral richer snippet).
• Harder golden set: negative queries scored separately in bench_recall (measure abstention without corrupting positive aggregates). Convex fusion evaluated → deferred.

Hardening

• Go daemon: fix service Start/Stop goroutine + watcher leak (cancellable ctx), watch newly-created subdirectories, stop debounce timer on exit; new lifecycle tests.
• Node MCP bridge: guard JSON.parse, clearer errors + Python-missing hint, advertise the real package version; accurate vault_fts_search description.
• CI: prettier now covers .mjs/.js/.cjs/.ts (formatted once), test-node on Linux+Windows+macOS, go test -race on Linux.

Docs / surface

New ADR-0026/0027/0028; ARCHITECTURE + CHANGELOG; new knobs surfaced in AGENTS.md / .agents rules / installed memory-rules block (regenerated, no drift). All six version markers → 3.9.0.

Honest notes

• New levers are situational by design (type-weighting/MMR are ~neutral on the generic single-relevant fixture; they help richly-typed / hub / topically-redundant vaults) — which is why they ship opt-in.
• Reranker quality depends on model–language fit (an English ms-marco model lowered recall on the Spanish fixture); the default is multilingual.

Verification

All CI-exact gates green locally: go test, pytest (103), npm test (34+50), version:check, sync-agents:check, prettier, markdownlint, linkcheck (110), retrieval-bench gate.

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