Implement ledger metrics for observability and add tests for Ruby packages endpoints

- Added `LedgerMetrics` class to record write latency and total events for ledger operations.
- Created comprehensive tests for Ruby packages endpoints, covering scenarios for missing inventory, successful retrieval, and identifier handling.
- Introduced `TestSurfaceSecretsScope` for managing environment variables during tests.
- Developed `ProvenanceMongoExtensions` for attaching DSSE provenance and trust information to event documents.
- Implemented `EventProvenanceWriter` and `EventWriter` classes for managing event provenance in MongoDB.
- Established MongoDB indexes for efficient querying of events based on provenance and trust.
- Added models and JSON parsing logic for DSSE provenance and trust information.

docs/modules/excititor/README.md

@@ -4,6 +4,7 @@ Excititor converts heterogeneous VEX feeds into raw observations and linksets th
 
 ## Latest updates (2025-11-05)
 - Link-Not-Merge readiness: release note [Excitor consensus beta](../../updates/2025-11-05-excitor-consensus-beta.md) captures how Excititor feeds power the Excititor consensus beta (sample payload in [consensus JSON](../../vex/consensus-json.md)).
+- Added [observability guide](operations/observability.md) describing the evidence metrics emitted by `EXCITITOR-AIAI-31-003` (request counters, statement histogram, signature status, guard violations) so Ops/Lens can alert on misuse.
 - README now points policy/UI teams to the upcoming consensus integration work.
 - DSSE packaging for consensus bundles and Export Center hooks are documented in the [beta release note](../../updates/2025-11-05-excitor-consensus-beta.md); operators mirroring Excititor exports must verify detached JWS artefacts (`bundle.json.jws`) alongside each bundle.
 - Follow-ups called out in the release note (Policy weighting knobs `POLICY-ENGINE-30-101`, CLI verb `CLI-VEX-30-002`) remain in-flight and are tracked in `/docs/implplan/SPRINT_200_documentation_process.md`.