fix: keep primary model after subagent runs

[sethcarlton]

Summary

Prevents subagent model from overriding primary agent model in TUI.

Fixes #8946
Fixes #7430

Reasoning

The existing isPrimaryAgent guard already prevents subagent names from overriding the primary agent selection. Model and variant appear to have been unintentionally left outside this guard, causing the leak. This fix moves them inside the same guard for consistency.

There is an existing PR #7428 that proposes a config option to opt-in to isolation. This would work but I think this is a bug and not a feature?

Verification

Tested according to repro steps in #8946: configured subagent with different model, invoked it, interrupted during work, navigated back to primary agent, the primary agent model was retained.

Note: the web app (packages/app/src/pages/session.tsx) may have the same bug but is not addressed here.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Related PRs Found

PR #7428 - feat: add config setting to avoid subagents models overriding session model

• feat: add config setting to avoid subagents models overriding session model #7428
• Why related: This PR is directly mentioned in the description as an alternative approach. PR fix: keep primary model after subagent runs #8951 takes a different stance, treating it as a bug fix rather than an opt-in feature.

PR #7156 - feat: add variant support for subagents

• feat: add variant support for subagents (#7138) [alt of #7140] #7156
• Why related: Addresses variant support for subagents, which relates to the variant model isolation issue mentioned in the current PR.

[rekram1-node]

thx!