Implement session ID tracking for Coding Agent launches

[oai-coding-agent]

Summary

• Added session telemetry that generates a UUIDv4 per launch, records start/end timestamps and duration, and persists events for basic usage analytics.

Implementation Details

• Introduced oai_coding_agent/telemetry/session.py:
  • SessionTracker manages a single launch lifecycle.
  • Generates a UUIDv4 session_id at start and writes a session_start JSONL record.
  • Persists a stable anonymized user_id (random UUID written once to XDG data dir user.json) to associate relaunches to the same user without PII.
  • On shutdown, writes a session_end record with ended_at and duration_ms. Also registers an atexit hook for extra safety.
  • Events are stored as JSON Lines at: XDG_DATA_HOME/oai_coding_agent/sessions.jsonl.
• CLI integration (cli.py):
  • Starts a session after RuntimeConfig is created, passing repository path, model, mode, repo and branch metadata when available.
  • Ensures the session is ended in a finally block after the console run (normal exit or KeyboardInterrupt), so end records are captured during tests and real runs.
• Event shape (one JSON object per line):
  • { event: "session_start", session_id, user_id, started_at, pid, version, python, repo_path?, model?, mode?, github_repo?, branch_name? }
  • { event: "session_end", session_id, user_id, ended_at, duration_ms, exit_reason? }

Assumptions Made

• Local JSONL persistence satisfies "persist to local file or telemetry backend"; a future backend can consume/ship these events.
• A random UUID stored once locally is sufficient as an anonymized user identifier to associate relaunches to the same user.
• Session tracking is scoped to agent launches (default/headless modes), not auxiliary subcommands (e.g., github login/logout).

Testing

• Added tests/telemetry/test_session.py:
  • Verifies SessionTracker writes start and end events with matching session_id and non-negative duration.
  • Verifies anonymized user_id is stable across tracker instances (simulated relaunch).
  • Verifies CLI run produces start and end events in sessions.jsonl.
• Ran project checks:
  • uv run ruff check && uv run ruff format
  • uv run mypy . (Success: no issues)
  • uv run pytest (156 passed, 1 skipped locally)

Considerations

• Chose append-only JSONL to be resilient (no in-place file updates) and easy to ingest in log pipelines. A single-file JSON map was considered but is less robust to concurrency/failures.
• The tracker currently uses atexit for best-effort end recording; the CLI also explicitly ends the session to ensure completeness.

closes #158