[feat] SQLite index for artifact queries + FTS

[NoobyGains]

Context

Git refs are the source of truth, but they are not queryable. Per docs/design.md "Layer 1", we need fast filters ("open Claims by agent X touching path Y", "unresolved Disagreements", "Plans pending objection") and full-text search across artifact bodies. SQLite is the projection layer.

Desired behavior

A @quorum/index-sqlite package providing:

• hydrate() — full rebuild by walking refs/coord/*.
• apply(event) — incremental update from a watchdog event ([feat] Define JSON schemas (Zod) for all 12 artifact types #8).
• query(filter): Artifact[] — typed filter (kind, author, status, paths[], parents-of, children-of, since).
• search(text, opts): { artifact, snippet }[] — FTS5 over body + title.
• Schema migrations versioned and idempotent.

Acceptance criteria

• FTS5 virtual table covering all text-bearing fields.
• hydrate() is idempotent and matches incremental apply() byte-for-byte.
• Query latency < 50ms p95 on 10k artifacts.
• Unit tests pass.
• Integration test against real git repo (no mocks) — write 1000 artifacts via chore(ci)(deps): Bump actions/checkout from 4 to 6 #2, hydrate, query, verify counts.

Implementation notes

• better-sqlite3 for sync API + perf.
• Treat the DB as a cache: deletable + rebuildable from refs at any time.
• Depends on chore(ci)(deps): Bump actions/setup-node from 4 to 6 #1, chore(ci)(deps): Bump actions/checkout from 4 to 6 #2. Feeds [infra] Set up CI matrix (Windows, macOS, Linux × Node 20/22) #4 (MCP query tools) and [feat] MCP stdio server stub responding to ping #9 (notification routing).

[NoobyGains]

Closed by 19d19ac — SQLite index + FTS5 shipped alongside git-refs store.