[BUG]: Primary-agent switches silently keep the previous agent's model in V2

Description

In the OpenCode 2 beta, switching primary agents changes the prompt and permissions but silently keeps the existing session model and variant.

For example, with Plan configured for provider/model-a#high and Build configured for provider/model-b#max, a Plan session remains on model A after Shift-Tab selects Build. The next request runs the Build agent on model A. Delegated children do use their own configured models, so the behavior is inconsistent between primary-agent switching and delegation.

This recreates the UX reported in #3550 and previously fixed by #3572. V2 now documents the separate session model, but the result is still surprising: an agent profile combines a prompt, permissions, and a model preference, and the UI presents the agent and model together.

This can silently run a role on a weaker, more expensive, or incompatible model and reasoning variant. It can also defeat an intended local-model boundary when a sensitive role is selected after a hosted-model role.

There is a legitimate opposite requirement in #41221: a model chosen manually should not be discarded on every agent switch. An unconditional "always switch" or "always preserve" rule makes one workflow fail.

Suggested behavior:

• Track whether the session model is following the agent default or pinned by the user.
• A new session initialized from an agent's configured model starts in follow-agent mode.
• In follow-agent mode, switching primary agents applies the new agent's configured model and variant.
• Selecting /models or /variants explicitly pins the session choice, so it survives later agent switches.
• Show when a session override is pinned and provide a simple "Use agent default" action to return to follow-agent mode.
• Update the selected agent and effective model atomically so plugins do not need timing-sensitive synchronization hooks.

That preserves explicit user choices while making configured role defaults behave like defaults. It prevents silent model/role mismatches without forcing plugin authors to build parallel preference stores or race the TUI.

Plugins

None required. The behavior reproduces with native V2 agents.<role>.model configuration.

OpenCode version

opencode2 v0.0.0-beta-17577

Steps to reproduce

1. Configure two selectable primary agents with different native model values and variants.
2. Start a new session with the first agent and confirm its configured model is active.
3. Shift-Tab to the second agent.
4. Send a prompt.
5. Observe that the second agent's prompt/permissions run with the first agent's session model instead of its configured model.
6. As a control, delegate to a child with its own configured model and observe that the child uses its configured model.

Screenshot and/or share link

No public share link; the session contains private repository context.

Operating System

Fedora Linux 44

Terminal

OpenCode TUI on Linux