feat: metrics bridge — hooks write ork-metrics-{session}.json

[yonatangross]

Phase 2b: Metrics Bridge

Hooks that track session state write to a shared metrics file that the statusline reads.

File format

{
  "session_id": "abc123",
  "dirty_files": 12,
  "alert_count": 2,
  "last_commit_ts": 1712345678000,
  "context_pct": 73
}

Path

/tmp/ork-metrics-{session_id}.json (same pattern as existing ork-ctx-pct-{session}.txt)

Hooks that write

• dirty-file-tracker.ts → dirty_files count
• commit-nudge.ts → last_commit_ts
• context-exhaustion-warner.ts → context_pct
• Any hook emitting alerts → increments alert_count

Implementation

• Use atomicWriteSync (already exists in lib/)
• Read-modify-write pattern (read existing, merge, write back)
• Cleanup: SessionEnd hook deletes the file

Acceptance criteria

• Metrics file written by relevant hooks
• atomicWriteSync prevents corruption
• Statusline reads it (from feat: ork-statusline.mjs — native statusline command #1294)
• SessionEnd cleanup

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