fix: Remove unsafe ClearDirtyIssues() method (bd-b6xo)

Remove ClearDirtyIssues() which had a race condition that could lose
dirty issues if export failed partway through. All callers now use
ClearDirtyIssuesByID() which only clears specific exported issues.

- Remove from Storage interface
- Remove from SQLite and Memory implementations
- Update 6 test call sites to use ClearDirtyIssuesByID()

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/storage.go

@@ -126,7 +126,6 @@ type Storage interface {
 	// Dirty tracking (for incremental JSONL export)
 	GetDirtyIssues(ctx context.Context) ([]string, error)
 	GetDirtyIssueHash(ctx context.Context, issueID string) (string, error) // For timestamp-only dedup (bd-164)
-	ClearDirtyIssues(ctx context.Context) error                            // WARNING: Race condition (bd-52), use ClearDirtyIssuesByID
 	ClearDirtyIssuesByID(ctx context.Context, issueIDs []string) error
 
 	// Export hash tracking (for timestamp-only dedup, bd-164)