feat: add custom tool description CLI arguments

[janbam]

Summary

This PR adds support for customizing the Self tool's description via CLI arguments, enabling users to tailor the tool description to their specific use case or deployment context.

Changes:

• ✅ New --tool-description <text> argument for inline custom descriptions
• ✅ New --tool-description-file <path> argument for file-based descriptions
• ✅ Mutual exclusivity validation between both arguments
• ✅ Duplicate usage detection with specific error messages
• ✅ Empty description validation (after trim)
• ✅ Comprehensive file reading error handling
• ✅ Updated help text and documentation

Implementation Details:

• Follows existing parseArgs validation patterns exactly
• Fail-fast philosophy: invalid configs prevent server startup
• Synchronous file reading for simplicity
• ~50 lines of code added total (KISS principle)

Files Modified:

• src/index.ts - Core implementation with CLI parsing and validation
• README.md - Documentation and usage examples
• CLAUDE.md - Developer notes and recent changes

Test Plan

All validation scenarios tested:

• Mutual exclusivity error when both flags used
• Duplicate flag usage detection
• Empty description validation
• Missing file error handling
• Empty file error handling
• Valid inline description works correctly
• Valid file-based description works correctly
• Custom descriptions appear correctly in MCP tool definition (verified via MCP-Debug)
• Default description used when neither flag provided