Why: U1 I3 needs a DB-side instance identity anchor matching
manifest.instance_id so wrong/missing DB blocks startup (I4).
Evidence: go test ./internal/store/ ./internal/app/; CLI smoke
ensure/verify on SQLite temp dir.
Impact: migration v26; store Ensure/Verify; identity subcommand;
serve checks AGENT_SYSTEM_INSTANCE_ID when set.
Refs: docs/recovery/u1-identity-manifest.md §5.2; Dev feature/recovery-u1-db-identity
Co-authored-by: Cursor <cursoragent@cursor.com>