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/memory/memory_test.go

@@ -698,8 +698,8 @@ func TestDirtyTracking(t *testing.T) {
 	}
 
 	// Clear dirty
-	if err := store.ClearDirtyIssues(ctx); err != nil {
-		t.Fatalf("ClearDirtyIssues failed: %v", err)
+	if err := store.ClearDirtyIssuesByID(ctx, dirty); err != nil {
+		t.Fatalf("ClearDirtyIssuesByID failed: %v", err)
 	}
 
 	dirty, err = store.GetDirtyIssues(ctx)