Background subagent completion flips active TUI model without viewing child session

Background subagent completion changes the active TUI model without opening the child session.

Repro:

1. Open a TUI parent session on opencode/muse-spark-1.3-contributor-free, variant xhigh.
2. Launch a background general subagent on opencode/mimo-v2.5-free.
3. Do not open the child session view.
4. Wait for the background completion notice, then check the TUI footer before opening anything.

Expected:
Parent stays on muse-spark-1.3-contributor-free.

Actual:
Active TUI model flips to mimo-v2.5-free.

Stored rows after the flip:

• parent ses_10a529466ffekP3eyrKuFbDs4t: build, {"id":"muse-spark-1.3-contributor-free","providerID":"opencode","variant":"xhigh"}
• child ses_f8d6a7bcfffefDiTWqKnKiNMIU: general, {"id":"mimo-v2.5-free","providerID":"opencode","variant":"default"}

So the persisted parent session keeps the right model; the live TUI selection follows the finished background child.

Related:

• #8946 / #8951 fixed the navigate-back variant by keeping model/variant updates inside the primary-agent guard.
• This is the background-completion variant: no child navigation happened before the flip.

Environment:

• opencode 1.18.25 TUI
• Latest release at report time: v1.18.29
• Linux, local config with explicit agent.build and subagent models.