Add Pinned field to Issue struct and database schema to protect
issues from accidental deletion via cleanup or compaction.
Changes:
- Add Pinned bool field to types.Issue
- Create migration 023_pinned_column.go for database schema
- Filter out pinned issues in cleanup command before deletion
- Add pinned check to GetTier1Candidates and GetTier2Candidates
- Add pinned check to CheckEligibility for compaction
- Update all SQL queries and scan functions to include pinned field
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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