feat: support all Freebuff models

[LuneZ99]

Adds the five current Freebuff picker models and their matching base3 agents.

Switching models now ends the model-locked Freebuff session before opening the next one. Freebuff requests are serialized across the full run/stream lifecycle so concurrent model changes cannot invalidate an in-flight request.

Checks:

• go build -o /tmp/cliproxy-freebuff ./cmd/server
• go test -run ^TestFreebuffRequestLifecycle$ ./internal/runtime/executor
• go test ./... (existing environment failures: 3 Claude OS fingerprint assertions and 1 WebRTC ICE gathering test)