feat: enhance changelog generation with incremental caching and improved AI summarization

## CHANGES

- Add incremental processing for new Git tags since cache
- Implement `WalkHistorySinceTag` method for efficient history traversal
- Cache new versions and commits after AI processing
- Update AI summarization prompt for better release note formatting
- Remove conventional commit prefix stripping from commit messages
- Add custom patterns directory support to plugin registry
- Generate unique patterns file including custom directory patterns
- Improve session string formatting with switch statement

internal/plugins/db/fsdb/sessions.go

@@ -86,9 +86,10 @@ func (o *Session) String() (ret string) {
 		ret += fmt.Sprintf("\n--- \n[%v]\n%v", message.Role, message.Content)
 		if message.MultiContent != nil {
 			for _, part := range message.MultiContent {
-				if part.Type == chat.ChatMessagePartTypeImageURL {
+				switch part.Type {
+				case chat.ChatMessagePartTypeImageURL:
 					ret += fmt.Sprintf("\n%v: %v", part.Type, *part.ImageURL)
-				} else if part.Type == chat.ChatMessagePartTypeText {
+				case chat.ChatMessagePartTypeText:
 					ret += fmt.Sprintf("\n%v: %v", part.Type, part.Text)
 				}
 			}