Resume sessions on launcher restart

[meawoppl]

Summary

• Store session_id in ExpectedSession config so it persists across launcher restarts
• When relaunching expected sessions, pass the stored session_id as resume_session_id so the proxy resumes with --resume instead of creating a fresh session
• Update the stored session_id after each successful spawn

Launcher-only fix — no protocol, backend, or migration changes. Existing configs without session_id get None (backward compatible) and start persisting after their next launch.

Test plan

• Launch a session via the portal, verify launcher.toml now has a session_id field
• Restart the launcher service, verify the session resumes (same session in UI) instead of creating a new one
• Verify scheduled tasks still work independently
• Verify clean exit (code 0) still removes the session from config