Persist pre_fix_sha to database for crash recovery

[chanakyav]

In orchestrator.py, _pre_fix_sha is stored in-memory on the orchestrator object:

self._pre_fix_sha = get_head_sha(self.task["branch"])

If the orchestrator crashes between FIX and VERIFY_PUSH, the SHA is lost. On restart, _do_verify_push() falls through with pre_sha = None and can't verify commits were actually pushed.

Proposed fix:

1. Add pre_fix_sha TEXT column to the tasks table (schema migration)
2. Store via update_task(task_id, pre_fix_sha=sha) in _do_fix()
3. Read from self.task["pre_fix_sha"] in _do_verify_push()
4. Follow the migration pattern: bump SCHEMA_VERSION, add to _MIGRATIONS, SCHEMA, _TASK_COLUMNS