fix(tui): prevent subagent sessions from overwriting the selected agent

[Haohao-end]

Summary

Fixes #18404

Prevent child/subagent sessions from mutating the globally selected primary agent in the TUI.

This change gates both:

• prompt-state restore
• tool-driven plan/build agent switching

to root sessions only (!session.parentID).

As a result, subagent sessions no longer overwrite the selected primary agent after tool activity, while existing root-session behavior remains unchanged.

Testing

• Verified the change is limited to:
  • packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
  • packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
• CI bun typecheck currently fails in unrelated packages/app files
• Local Bun/tsgo validation was unavailable in the original shell

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[Haohao-end]

Updated the PR description to use the template and linked the issue with Fixes #18404.

Also, the current bun typecheck failure appears to be in packages/app:

• src/components/dialog-connect-provider.tsx
• src/components/dialog-custom-provider.tsx

Those files are unrelated to this PR’s TUI-only changes.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.