fix(agent): fall back to an available project agent

[khrypchenko]

Summary

• validate the stored agent against the selected project's available primary agents before opening a session, sending a prompt, or refreshing the keyboard
• fall back to build (or the first available primary agent) and persist that choice so project switches do not send unsupported agent names to OpenCode
• add regression coverage for project-specific agent fallback behavior and update command tests for the new resolver

Verification

• npm run build
• npm run lint
• npm test