Fixed two code paths where pinned=false from JSONL would overwrite
existing pinned=true in database:
- importer.go: Only update pinned if explicitly true in JSONL
- multirepo.go: Use COALESCE to preserve existing pinned value
Added tests for pinned field preservation.
Note: Bug may have additional code path - investigation ongoing.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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