The session-listing path already opens the live agent state.db read-only
(file:...?mode=ro) so a write-capable handle doesn't add checkpoint/lock
surface while the agent streams into the same WAL DB (#5455). Three pure-read
projections were missed and still opened a read-WRITE connection:
- read_session_lineage_report
- read_session_lineage_metadata
- the gateway-watcher fingerprint projection (a 5s poll)
Route all three through a shared open_state_db_readonly() helper that mirrors
the listing path: read-only file: URI with a writable fallback that warns. The
self-heal write path (missing idx_messages_session) is intentionally left
writable. No behavior change beyond open mode; reads perform zero writes.
Refs #5455
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
