feat(tools): implement issue operations - Closes #3

Implement Gitea issue operations tools with the following features:
- gitea_list_issues: List issues with filters (state, labels, milestone)
- gitea_get_issue: Get single issue by number
- gitea_create_issue: Create new issue with title, body, labels, milestone
- gitea_update_issue: Update issue state, title, body, labels, assignees

Files added:
- src/gitea_mcp/tools/issues.py: Issue operation tool handlers

Files modified:
- src/gitea_mcp/server.py: Register issue tools in MCP server
- src/gitea_mcp/tools/__init__.py: Export issue tool functions

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

src/gitea_mcp/tools/__init__.py

@@ -1 +1,8 @@
 """Gitea MCP tools package."""
+
+from .issues import get_issue_tools, handle_issue_tool
+
+__all__ = [
+    "get_issue_tools",
+    "handle_issue_tool",
+]