feat: metrics record --emit-event for kaizen.metrics.recorded

Publish activity-core EventEnvelope payloads to NATS subject
activity.kaizen.metrics.recorded after a successful append.
Optional nats-py via kaizen-agentic[events]; project slug from
KAIZEN_PROJECT_SLUG or directory basename. Skips emit on
idempotency duplicates. Closes KAIZEN-WP-0008 T03.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- **`metrics record --emit-event`** — publishes `kaizen.metrics.recorded` NATS
+  envelope for activity-core event-driven definitions (optional `nats-py` via
+  `pip install 'kaizen-agentic[events]'`)
+- **Event contract** — `docs/integrations/kaizen-metrics-recorded-event.md`
+
 ## [1.4.0] - 2026-06-18
 
 ### Added