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>

server.py

@@ -61,7 +61,7 @@ server: Server = Server("gemini-server")
 TOOLS = {
     "think_deeper": ThinkDeeperTool(),  # Extended reasoning for complex problems
     "review_code": ReviewCodeTool(),  # Comprehensive code review and quality analysis
-    "debug_issue": DebugIssueTool(),  # Root cause analysis and debugging assistance
+    "debug": DebugIssueTool(),  # Root cause analysis and debugging assistance
     "analyze": AnalyzeTool(),  # General-purpose file and code analysis
     "chat": ChatTool(),  # Interactive development chat and brainstorming
     "review_changes": ReviewChanges(),  # Pre-commit review of git changes