feat: add persistent dashboard service

[douglasmonsky]

Summary

• add a macOS LaunchAgent lifecycle for a persistent localhost dashboard
• reserve 127.0.0.1:47821 by default and refuse unknown port collisions
• add dashboard-service install, status, and uninstall commands
• start from the cached aggregate index so the service binds promptly
• add a lightweight health endpoint, lifecycle rollback coverage, and user documentation

User impact

macOS users can install the dashboard once and reuse a stable local URL across terminal and Codex task lifetimes. The service starts at login, restarts after failure, remains loopback-only, and never opens a browser automatically.

Review

No correctness, security, or privacy blockers found. The service validates its managed plist, refuses occupied ports, rolls back failed updates, and leaves raw-context access behind the existing per-server token boundary.

Residual operational constraint: the LaunchAgent references the Python interpreter used at installation time, so moving or deleting that environment requires reinstalling the service.

Validation

• python -m pytest -q — 1,114 passed
• python -m ruff check .
• python -m mypy
• python scripts/check_release.py
• git diff --check origin/main...HEAD
• live LaunchAgent status and http://127.0.0.1:47821 HTTP 200 probe