Add Pi provider integration and improve git status handling

[Emanuele-web04]

Summary

• Add Pi as a supported provider, including new server adapters, provider/session lifecycle updates, and UI model selection changes.
• Refactor shared model/provider contracts and discovery logic to account for Pi-specific capabilities and compatibility rules.
• Improve git status caching and refresh behavior so local file changes update independently from cached remote metadata.
• Surface friendlier pull failures when local changes would block git pull.
• Update settings, sidebar, composer, and related web flows to reflect the new provider model and trait handling.

Testing

• Added and updated unit/integration coverage for Pi provider behavior, git status refresh/caching, model compatibility, and thread/session flows.
• Verified the new pull error path with a test that reports blocking local files.
• Not run: full workspace verification (bun fmt, bun lint, bun typecheck).