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_server.py

@@ -21,7 +21,7 @@ class TestServerTools:
         # Check all core tools are present
         assert "think_deeper" in tool_names
         assert "review_code" in tool_names
-        assert "debug_issue" in tool_names
+        assert "debug" in tool_names
         assert "analyze" in tool_names
         assert "chat" in tool_names
         assert "review_changes" in tool_names