Add CheckSyncDivergence doctor check that detects:
- JSONL on disk differs from git HEAD version
- SQLite last_import_time does not match JSONL mtime
- Uncommitted .beads/ changes exist
Each issue includes auto-fix suggestions (bd sync, bd export, git commit).
Multiple divergence issues result in error status.
Part of GH#885 recovery mechanism.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>