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

[grinev]

@khrypchenko thanks for the fix - the new agent resolver looks good, and the regression tests around project-specific fallback are useful.

I found one blocking gap before merge: the existing session selection flow still does not validate the stored agent against the selected project. In src/bot/commands/sessions.ts, the keyboard is initialized and sent after switching to an existing session, but this path does not call resolveProjectAgent() or otherwise refresh the agent state for the current project.
Because of that, if the user has an agent saved from another project, the bot can still show an unsupported agent in the reply keyboard after opening an existing session. This seems to be part of the behavior described in the PR ("before opening a session" / "refreshing the keyboard"), so I think this path should be covered as well.

Could you please apply the same normalization here as you already did in /new, project switching, prompt sending, model change, and variant change? It would also be good to add a regression test for the existing-session flow.