Add Prime Agent as an ACP provider

[Bradley-Butcher]

Summary

Add Prime Agent as a known ACP provider.

When prime-agent is installed, bb detects it automatically and shows it in the provider picker with its models, thinking levels, and butterfly mark.

This supports the standard bb thread loop: prompts, streamed responses and reasoning, tool calls, cancellation, and images.

Addresses #1078

Scope

This change uses Prime's ACP interface. It does not add a dedicated Prime RPC bridge or bb controls for Prime-specific features such as goals, autonomous runs, schedules, heartbeats, agent messaging, or session management.

Those features require a separate RPC integration.

Implementation

Prime's ACP session does not expose model discovery or model selection. bb reads prime-agent model list and passes the selected model and thinking level when it starts Prime.

bb starts Prime with --no-session because bb owns the thread history. Provider-qualified model IDs remain distinct when different Prime providers offer a model with the same name.

Validation

• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/server --filter=@bb/agent-runtime
• 156 server and ACP bridge tests pass
• live threads start with openai-codex/gpt-5.6-luna, openai-codex/gpt-5.6-sol, and openai-codex/gpt-5.6-terra

[Bradley-Butcher]

Sorry just read your contributing guidelines! Feel free to close if there is no desire for this PR