Add excel-to-markdown skill and converter script

- Add excel_to_markdown.py script for converting Excel files to Markdown tables
- Create excel-to-markdown skill with SKILL.md documentation
- Support automatic data range detection and flexible output options
- Include features for row/column numbering and partial conversions
- Add skill-creator reference documentation

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

.gitignore

@@ -0,0 +1,43 @@
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# UV
+.venv/
+uv.lock
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Project specific
+*.xlsx
+*.xls
+
+# .claude
+.claude/