refactor: rename debug_issue tool to debug for brevity

- Rename debug_issue.py to debug.py
- Update tool name from 'debug_issue' to 'debug' throughout codebase
- Update all references in server.py, tests, and README
- Keep DebugIssueTool class name for backward compatibility
- All tests pass with the renamed tool

This makes the tool name shorter and more consistent with other
tool names like 'chat' and 'analyze'. The functionality remains
exactly the same.

🤖 Generated with Claude Code

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

tools/__init__.py

@@ -4,7 +4,7 @@ Tool implementations for Gemini MCP Server
 
 from .analyze import AnalyzeTool
 from .chat import ChatTool
-from .debug_issue import DebugIssueTool
+from .debug import DebugIssueTool
 from .review_changes import ReviewChanges
 from .review_code import ReviewCodeTool
 from .think_deeper import ThinkDeeperTool