Session lifecycle: session ID, start, stop, elapsed time

[Omoshirokunai]

What

When the extension loads on a Google Docs tab (a session starts).
A session has a UUID, a start timestamp, and an elapsed timer.

Acceptance criteria

• sessionId generated as UUID v4 on tab load, stored in chrome.storage.local
• startedAt recorded as ISO 8601 timestamp
• Elapsed time readable from storage (for popup display)
• Session data cleared on explicit Stop or new session start
• No session ID is ever reused

Notes

Session ID will appear in the exported process-log.json.
See TWFF spec (metadata.json and session_id field).