docs: update MCP tool description guidelines based on lessons learned

Changed "Context Efficiency" section to prioritize LLM usability over
brevity. Key points:
- LLM's ability to use tools correctly is the top priority
- Complex tools require detailed descriptions for correct usage
- Token cost is acceptable if it ensures LLM doesn't misuse tools
- Avoid verbose phrasing, but never sacrifice clarity for brevity

This lesson came from attempting to shorten the sharepoint_excel tool
description, which caused LLM to provide incorrect responses. The
original detailed description was necessary for proper functionality.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: k-ibaraki

CLAUDE.md

@@ -89,9 +89,12 @@ def some_function():
 - [ ] Does this add significant value to justify the complexity?
 
 **Context Efficiency**
-- Keep tool descriptions concise (they're included in every LLM call)
-- Prioritize essential information over exhaustive documentation
-- Use external docs (README) for detailed explanations
+- **LLM Usability First**: Tool descriptions must prioritize LLM's ability to use the tool correctly
+- Avoid verbose or redundant phrasing, but never sacrifice clarity for brevity
+- Include essential information that LLM needs for correct usage (modes, defaults, behavior)
+- Complex tools require detailed descriptions - token cost is acceptable if it ensures correct usage
+- Use external docs (README) for human-oriented explanations, not as replacement for tool clarity
+- **Lesson learned**: Overly brief descriptions can cause LLM to misuse tools, negating any token savings
 
 ### Project-Specific Guidelines