feat: add search data layer — adapter, highlight helper, hook, and tests

docs: refresh README and TODO after search+opencode merge

README: add search, projects, stats, saved-searches to API surface table;
remove stale reference to unshipped search task
TODO: mark #3 reviewed, unblock #7

search: add /api/v1/search API and opencode collector parser

- Add FTS5 search repository with BM25 ranking, cursor pagination, and owner scoping
- Expose GET /api/v1/search with query, tool, host, since/until filters
- Add opencode collector parser reading from SQLite opencode.db
- Spike document recording canonical opencode storage format

Invariants: no schema migrations, no wire type changes, read-only search,
marker snippets (not HTML), invalid queries → 400 INVALID, parser interface
unchanged, collector state schema preserved, stats/react stubs untouched.

Reviewed: offset fix (message.rowid instead of time_created for progress marker),
all 22 test packages pass.

docs: refresh search and opencode plan

docs: conclude lethe collector task

collector: persist skipped-only parser progress

docs: record collector endpoint and outbox checks

collector: enforce outbox cap before replay

docs: record collector final fixes

collector: fix daemon drain and backfill start

docs: update collector verify notes

collector: preserve valid rows around ingest errors

collector: skip rejected rows after partial ingest

collector: add lethe-collector cli

collector: add polling source runner

collector: align ingest sender with server response

collector: add ingest sender and outbox replay

collector: add config and state store

docs: clarify collector plan assumptions

docs: plan lethe collector implementation