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.go

@@ -1313,14 +1313,6 @@ func (m *MemoryStorage) GetDirtyIssues(ctx context.Context) ([]string, error) {
 	return dirtyIDs, nil
 }
 
-func (m *MemoryStorage) ClearDirtyIssues(ctx context.Context) error {
-	m.mu.Lock()
-	defer m.mu.Unlock()
-
-	m.dirty = make(map[string]bool)
-	return nil
-}
-
 func (m *MemoryStorage) ClearDirtyIssuesByID(ctx context.Context, issueIDs []string) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()