fix(plan): don't carry plan agent model into build on plan_exit - #50609

Issue for this PR

Closes #9296

Note: #9296 was closed pending v2 and the bug still reproduces on dev. Earlier attempts #23971 and #29457 were closed unmerged.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

plan_exit resolved the model for the synthetic build user message from the last user message, which in plan mode carries the plan agent's model. The run loop (SessionPrompt runLoop) picks its model from the newest user message on every turn and never reads agent.build.model mid-loop, so with both agent.plan.model and agent.build.model configured, the build kept running the plan model.

This applies the same precedence createUserMessage uses when creating prompts (input.model ?? ag.model ?? currentModel): the build agent's configured model first, then the last user message's model, then the provider default. The build agent's variant is resolved the way createUserMessage resolves it (only when the agent's configured model was selected and the model exposes that variant).

How did you verify your code works?

• New test packages/opencode/test/tool/plan.test.ts: with agent.plan.model = test/test-model and agent.build.model = test/build-model, asserts plan_exit stamps build-model on the synthetic message (failed with test-model before the fix); a second case asserts the fallback to the plan model when the build agent has no configured model.
• bun test test/tool/plan.test.ts test/session/prompt.test.ts in packages/opencode: 61 pass, 1 skip, 0 fail.
• bun test test/agent/agent.test.ts test/agent/plan-mode-subagent-bypass.test.ts: 48 pass, 0 fail.
• bun typecheck in packages/opencode: clean.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR