Schema 3: opportunistic CI/SCM context.
Adds three low-cardinality, non-personal enum fields to the completion event, all sent clear:
- event_trigger: push / pull_request / schedule / manual / other / unknown
- runner_os: linux / macos / windows / unknown
- ref_kind: default-branch / branch / tag / unknown (coarsened; the raw branch or tag name is never sent)
All three are env-derived inside buildTelemetryEvent, so consuming actions need no source change. The collector accepts schema_version 1, 2, and 3; older bundles keep ingesting with the new fields defaulted to unknown. Strict no-PII posture preserved: no hashed user identifiers, no clear names.