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/sqlite/labels_test.go

@@ -351,9 +351,9 @@ func TestLabelMarksDirty(t *testing.T) {
 	}
 
 	// Clear dirty issues
-	err = store.ClearDirtyIssues(ctx)
+	err = store.ClearDirtyIssuesByID(ctx, []string{issue.ID})
 	if err != nil {
-		t.Fatalf("ClearDirtyIssues failed: %v", err)
+		t.Fatalf("ClearDirtyIssuesByID failed: %v", err)
 	}
 
 	// Add label - should mark issue dirty
@@ -372,9 +372,9 @@ func TestLabelMarksDirty(t *testing.T) {
 	}
 
 	// Clear dirty again
-	err = store.ClearDirtyIssues(ctx)
+	err = store.ClearDirtyIssuesByID(ctx, []string{issue.ID})
 	if err != nil {
-		t.Fatalf("ClearDirtyIssues failed: %v", err)
+		t.Fatalf("ClearDirtyIssuesByID failed: %v", err)
 	}
 
 	// Remove label - should mark issue dirty