1.2.5

Add new models: sonnet-4.5 and haiku-4.5 (#492)

* feat: Update chat models and implement migration from v11 to v12

- Updated default chat model ID to 'claude-sonnet-4.5' and added new recommended models.
- Introduced migration logic to transition settings from version 11 to 12, merging existing chat models with new defaults and adding 'claude-sonnet-4.5' and 'claude-haiku-4.5'.
- Removed outdated models: 'claude-sonnet-4.0', 'claude-3.7-sonnet', 'claude-3.5-sonnet', and 'claude-3.5-haiku'.
- Added tests to verify migration functionality.

* docs: Update README.md with maintenance notice and contribution invitation

- Added a maintenance notice indicating the plugin is not under active development and may have delayed responses to issues and feature requests.
- Included an invitation for community contributions and collaboration.