Append precise UTC timestamps to Rig logs

[Copilot]

Add a millisecond-precision UTC timestamp as the last field of every Rig JSONL log record.

• Log format: Append an ISO 8601 timestamp after type and optional data, including serialization-error fallback records.
• Coverage and docs: Add regression coverage for field ordering, successive timestamps, omitted data, and serialization failures; document the format.

{"type":"rig.agent:turn","data":{"turn":1},"timestamp":"2026-10-13T13:00:41.033Z"}

[pelikhan]

@copilot Use performance now

[Copilot]

Updated in 04cecab: timestamp now uses performance.now() as a numeric, high-resolution value and remains the last field. Tests and documentation updated.

[pelikhan]

@copilot add created_at that uses DateTime.now

[Copilot]

@copilot add created_at that uses DateTime.now

Added in 6255600: created_at uses JavaScript’s Date.now() (wall-clock milliseconds since the Unix epoch). The high-resolution performance.now() timestamp remains the last field. Tests and documentation updated.