Implement automatic migration of existing configuration from rover.json and .rover/settings.json to the new structure.

Parent task: #336
Depends on: Subtasks 3.2, 3.12

Details

Migration steps:

1. Detect existing rover.json in project root
2. Migrate relevant settings to <project>/.rover/settings.json (shared config)
3. Migrate user-specific settings to global config
4. Keep rover.json as deprecated but functional during transition
5. Print deprecation notice

Configuration mapping:

• Project-specific (languages, mcps, etc.) → .rover/settings.json
• User-specific (default agent, telemetry) → global config

TODO

• Implement detection of existing config files
• Create config migration logic with proper mapping
• Handle conflicts between old and new config
• Add deprecation warning for rover.json
• Document migration path for users
• Add tests for config migration