docs: comprehensive documentation improvements

- Add 'Gemini CLI Extension' to README title for better context and SEO
- Simplify installation with direct GitHub URL method as primary approach
- Clarify command parameter descriptions (filename-or-path options)
- Clean up .gitignore: remove duplicates, consolidate sections (244→164 lines)
- Eliminate redundant patterns and generic template sections
- Maintain project-specific ignore patterns while improving readability

Co-Authored-By: GLM <[email protected]>