chore: 🧹 remove fs_write.rs.bak and update tool descriptions

Removes an unused backup file (fs_write.rs.bak) and updates the tool descriptions in tool_index.json to improve guidance for fs_read and fs_write tools. Also updates the batch file operations RFC to reflect current implementation status.

- Removed: Deleted obsolete fs_write.rs.bak file
- Updated: Enhanced tool descriptions with better batching guidance
- Updated: RFC documentation to accurately reflect implementation status
- Added: Implementation status section to RFC with next steps

🤖 Assisted by [Amazon Q Developer](https://aws.amazon.com/q/developer)

Author: jsamuel1