[FEATURE]: Crash recovery — auto-resume sessions after crash

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When opencode crashes (SIGKILL, panic, OOM, power loss), sessions that were running are left in an indeterminate state. You can already recover them manually by finding the session ID and using --continue\ or --session, but you have to do this by hand for each session.

I'd like an opt-in config option that automatically detects the crash on next startup and resumes all sessions that were active at crash time — no manual lookup or --continue\ needed.