Two improvements for Dolt backend users with stale git hooks:
1. Migration warning: When running 'bd migrate --to-dolt', warn if
installed hooks lack the Dolt backend check and suggest reinstalling.
2. Doctor check: New CheckGitHooksDoltCompatibility() detects when
hooks exist but lack the Dolt skip logic, surfacing the error with
a clear fix message.
Both changes help users who migrated to Dolt but have pre-migration
hooks that try to run JSONL sync (which fails with Dolt backend).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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