feat(tools): add label and milestone operations

- Create labels.py with gitea_list_labels and gitea_create_label tools
- Integrate milestone tools from milestones.py into server
- Update tools/__init__.py with all tool exports
- Update server.py to handle label and milestone tool calls

Closes #4, Closes #5

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

src/gitea_mcp/tools/__init__.py

@@ -2,10 +2,13 @@
 
 from .issues import get_issue_tools, handle_issue_tool
 from .labels import get_label_tools, handle_label_tool
+from .milestones import get_milestone_tools, handle_milestone_tool
 
 __all__ = [
     "get_issue_tools",
     "handle_issue_tool",
     "get_label_tools",
     "handle_label_tool",
+    "get_milestone_tools",
+    "handle_milestone_tool",
 ]