[Feature]: Plugin install/load confirmation

### Feature Request

When loading a project, Rivet should prompt for confirmation to install and/or enable plugins. This will help with security, so that we don't load evil plugins without confirmation, and if a project uses something dangerous like `rivet-plugin-fs`, the user will be able to know about it.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

The issue names no files, tests, or entry points; start by locating the project-loading flow and the code that installs or enables plugins. Define confirmation for both installation and enabling, including how potentially dangerous plugins such as rivet-plugin-fs are shown, then add coverage for the expected prompts and refusal behavior.

Written by the indexing model from the issue text.