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_prompt_regression.py

@@ -12,7 +12,7 @@ import pytest
 
 from tools.analyze import AnalyzeTool
 from tools.chat import ChatTool
-from tools.debug_issue import DebugIssueTool
+from tools.debug import DebugIssueTool
 from tools.review_changes import ReviewChanges
 from tools.review_code import ReviewCodeTool
 from tools.think_deeper import ThinkDeeperTool
@@ -167,8 +167,8 @@ class TestPromptRegression:
                     assert output["status"] == "success"
 
     @pytest.mark.asyncio
-    async def test_debug_issue_normal_error(self, mock_model_response):
-        """Test debug_issue tool with normal error description."""
+    async def test_debug_normal_error(self, mock_model_response):
+        """Test debug tool with normal error description."""
         tool = DebugIssueTool()
 
         with patch.object(tool, "create_model") as mock_create_model: