Telemetry: Emitting events

[nikomatsakis]

Wire up the places that should produce telemetry events.

Part of the telemetry initiative — see sync discussions: June 9, June 23.

Milestones

• Add "Stop" / end-of-turn hook event — New hook event fired when the agent signals it has completed a turn. Initially minimal payload ("turn ended"); later attach summaries (files edited, tokens used, etc.).
• Activity metric events — Emit events for: skill activation (which skill, which crate triggered it), hook invocation (which hook, which plugin, duration), sync runs (skills installed/reaped, plugins matched), session start (agent type, workspace crate count).
• Outcome-oriented events (experimental) — "Churn" signal: detect repeated edits to the same file/region within a turn. Crate-correlated churn: tag churn events with which dependency is involved. Best-effort/heuristic, will need iteration.

Notes

• The Stop event is essential for the feedback collection workstream too.
• Activity metrics are straightforward; outcome metrics require experimentation.