Fix race condition in dirty issue tracking (bd-52, bd-53)

Fix critical TOCTOU bug where concurrent operations could lose dirty
issue tracking, causing data loss in incremental exports. Also fixes
bug where export with filters would incorrectly clear all dirty issues.

The Problem:
1. GetDirtyIssues() returns [bd-1, bd-2]
2. Concurrent CRUD marks bd-3 dirty
3. Export writes bd-1, bd-2
4. ClearDirtyIssues() deletes ALL (including bd-3)
5. Result: bd-3 never gets exported!

The Fix:
- Add ClearDirtyIssuesByID() that only clears specific issue IDs
- Track which issues were actually exported
- Clear only those specific IDs, not all dirty issues
- Fixes both race condition and filter export bug

Changes:
- internal/storage/sqlite/dirty.go:
  * Add ClearDirtyIssuesByID() method
  * Add warning to ClearDirtyIssues() about race condition
- internal/storage/storage.go:
  * Add ClearDirtyIssuesByID to interface
- cmd/bd/main.go:
  * Update auto-flush to use ClearDirtyIssuesByID()
- cmd/bd/export.go:
  * Track exported issue IDs
  * Use ClearDirtyIssuesByID() instead of ClearDirtyIssues()

Testing:
- Created test-1, test-2, test-3 (all dirty)
- Updated test-2 to in_progress
- Exported with --status open filter (exports only test-1, test-3)
- Verified only test-2 remains dirty ✓
- All existing tests pass ✓

Impact:
- Race condition eliminated - concurrent operations are safe
- Export with filters now works correctly
- No data loss from competing writes

Closes bd-52, bd-53

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

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

internal/storage/storage.go

@@ -45,7 +45,8 @@ type Storage interface {
 
 	// Dirty tracking (for incremental JSONL export)
 	GetDirtyIssues(ctx context.Context) ([]string, error)
-	ClearDirtyIssues(ctx context.Context) error
+	ClearDirtyIssues(ctx context.Context) error // WARNING: Race condition (bd-52), use ClearDirtyIssuesByID
+	ClearDirtyIssuesByID(ctx context.Context, issueIDs []string) error
 
 	// Config
 	SetConfig(ctx context.Context, key, value string) error