**Problem:**
Closed issues were silently reopening during git sync/import operations.
When importing an issue update, the importer built an updates map with
status='closed' but NO closed_at timestamp. The UpdateIssue() function's
manageClosedAt() would only set closed_at when status was CHANGING to
closed, not when it was already closed. Result: closed_at got cleared,
effectively reopening issues.
**Root Cause:**
1. Importer built updates map without closed_at field (lines 443-451, 519-528)
2. closed_at was not in allowedUpdateFields whitelist
3. manageClosedAt() only managed closed_at for status TRANSITIONS
4. Import of already-closed issue → closed_at lost → issue reopens
**Impact:**
- WASM issues (bd-44d0, bd-8507, etc.) were closed on Nov 4 (commit 0df9144)
- They reopened as 'open' status during sync on Nov 5 (commit 8c9814a)
- Users had to repeatedly close the same issues
- Data integrity violation: status=closed with closed_at=NULL
**Fix:**
1. Add closed_at to allowedUpdateFields whitelist
2. Add closed_at to importer updates maps (both external_ref and ID paths)
3. Update manageClosedAt() to skip auto-management if closed_at explicitly provided
- Preserves import timestamps while maintaining auto-management for CLI operations
**Testing:**
- All internal/importer tests pass
- All internal/storage/sqlite tests pass
- Explicitly tests timestamp preservation in TestImportWithExternalRef
**Files Changed:**
- internal/importer/importer.go: Add closed_at to updates maps
- internal/storage/sqlite/sqlite.go: Allow closed_at updates, respect explicit values
Amp-Thread-ID: https://ampcode.com/threads/T-53ed6e45-9d04-4a35-97e9-d1ec36321ab0
Co-authored-by: Amp <amp@ampcode.com>
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