Add 'kart mv' command

**Problem** Renaming a tracked layer is a common operation, but the current process is manual, verbose, and error-prone. It requires a multi-step workflow (`export`, `remove`, `import`) and the creation of temporary files. **Solution** Implement a new command, `kart mv`. * **Syntax:** `kart mv <old_name> <new_name>` * **Action:** The command should atomically rename the layer in the data source and update Kart's tracking metadata. The change should then be staged for the next commit. **Describe alternatives you've considered** The only alternative is the current manual workflow: 1. `kart export <old_name> --output temp.gpkg` 2. `rm <old_name>` 3. `kart import temp.gpkg <new_name>` 4. `rm temp.gpkg` **Additional context** This command would be the direct analog to `git mv`, significantly improving the user experience.