v2.9.0

feat(extensions): activate honker via vendored binary + discovery

Build libhonker_ext from source and vendor the darwin-arm64 binary so the
dormant Phase 8 honker paths (embedding queue, leader cron, pub/sub) go live
instead of no-op'ing to the pre-honker fallbacks.

- add locateHonkerExt() + honkerExtCandidates() discovery, mirroring the
  existing locateSystemSqlite()/systemSqliteCandidates() pattern
- flip wantHonker default false -> auto-on when a binary is discoverable;
  LTM_DISABLE_HONKER still force-disables, LTM_HONKER_EXT overrides path
- lib/honker.ts openHonker() uses discovery instead of env-only read
- extract firstExisting() helper, dedup the locate loops
- update extensions tests to the new contract (off via LTM_DISABLE_HONKER,
  on when vendored binary present)
- raise test --timeout to 15000 to kill the gdrive-FS load flake
- bump 2.8.1 -> 2.9.0 across all version refs