[Bug] /fork reverts to the CLI --model/--thinking instead of the model selected in the session

[israellot]

Summary

Forking a session (/fork, or the fork daemon/RPC command) does not keep the model and thinking level that were active in the source session. The forked session comes up with the model/thinking level from the CLI launch config (--model, --thinking) instead.

Repro

1. Start prime-agent --model <provider>/<model-A> (interactive or --mode rpc).
2. Switch to model B with /model (or set_model), optionally change /effort.
3. Send a prompt, then /fork from any earlier user message (or fork "at" the current position).
4. The forked session runs on model A again. The transcript still shows the earlier turns on model B.

It only reproduces when a --model was given at launch. Without it, createAgentSession restores the model from the transcript and it looks fine.

/resume and /new are not affected in the same way.

Root cause

AgentSessionRuntime.fork() in packages/coding-agent/src/core/agent-session-runtime.ts rebuilds the runtime with only sessionConfig: this.sessionConfig and no sessionOptions. createRuntime in main.ts then runs buildSessionOptions(config, ...), which resolves config.model / config.thinking from the startup config, and the explicit CLI model wins over the transcript's model_change entries in createAgentSession.

Still present on upstream/main at 3484f06a1.

Fix

Capture the live session's model, thinkingLevel, serviceTier, and scopedModels before the fork and pass them as sessionOptions on all three fork paths. resolveRuntimeSessionOptions already gives sessionOptions precedence over the CLI config, so no other changes are needed.

Patch with a regression test and a .changes fragment, rebased on upstream/main:
israellot@36284ef
(branch fix/fork-keeps-session-model, +50/-0 across 3 files)

Validation: the new test fails on main (Expected "faux-2", Received "faux-1") and passes with the patch; test/suite/agent-session-runtime.test.ts, test/daemon-mode.test.ts, test/interactive-mode-clone-command.test.ts, and the 2860 regression test pass; tsgo --noEmit and biome check are clean.

Happy to open the PR if a maintainer wants it.