Fix bd-pq5k: merge conflicts now prefer closed>open and deletion>modification

CHANGES:
1. Merge logic (internal/merge/merge.go):
   - Added mergeStatus() enforcing closed ALWAYS wins over open
   - Fixed closed_at handling: only set when status='closed'
   - Changed deletion handling: deletion ALWAYS wins over modification

2. Deletion tracking (cmd/bd/snapshot_manager.go):
   - Updated ComputeAcceptedDeletions to accept all merge deletions
   - Removed "unchanged locally" check (deletion wins regardless)

3. FK constraint helper (internal/storage/sqlite/util.go):
   - Added IsForeignKeyConstraintError() for bd-koab
   - Detects FK violations for graceful import handling

TESTS UPDATED:
- TestMergeStatus: comprehensive status merge tests
- TestIsForeignKeyConstraintError: FK constraint detection
- bd-pq5k test: validates no invalid state (status=open with closed_at)
- Deletion tests: reflect new deletion-wins behavior
- All tests pass ✓

This ensures issues never get stuck in invalid states and prevents
the insane situation where issues never die!

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/storage/sqlite/util.go

@@ -51,4 +51,15 @@ func IsUniqueConstraintError(err error) bool {
 	return strings.Contains(err.Error(), "UNIQUE constraint failed")
 }
 
+// IsForeignKeyConstraintError checks if an error is a FOREIGN KEY constraint violation
+// This can occur when importing issues that reference deleted issues (e.g., after merge)
+func IsForeignKeyConstraintError(err error) bool {
+	if err == nil {
+		return false
+	}
+	errStr := err.Error()
+	return strings.Contains(errStr, "FOREIGN KEY constraint failed") ||
+		strings.Contains(errStr, "foreign key constraint failed")
+}
+