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>

tests/test_thinking_modes.py

@@ -7,7 +7,7 @@ from unittest.mock import Mock, patch
 import pytest
 
 from tools.analyze import AnalyzeTool
-from tools.debug_issue import DebugIssueTool
+from tools.debug import DebugIssueTool
 from tools.review_code import ReviewCodeTool
 from tools.think_deeper import ThinkDeeperTool