feat(tui): show model variant in subagent footer

[samiralibabic]

Issue for this PR

Closes #26266

Type of change

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

What does this PR do?

Adds model, provider, and variant details to subagent sessions, similar to the main-agent footer.

A persisted non-default session variant is shown first. Legacy sessions fall back to a matching configured agent variant or supported reasoningEffort. Existing usage and navigation details stay unchanged.

How did you verify your code works?

• bun test test/cli/tui
• bun turbo typecheck
• Manual TUI verification

Screenshots / recordings

Before	After

Checklist

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

[github-actions]

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

Based on my search results, I found several related PRs that address similar concerns about model variants and subagent sessions, but no exact duplicate PRs:

Related but not duplicates:

• fix(session): agent.model and agent.variant config ignored in handleSubtask #36281 - fix(session): agent.model and agent.variant config ignored in handleSubtask - Addresses variant handling in subagent execution
• feat(agent): implement model tier selection with variant support for subagents #11377 - feat(agent): implement model tier selection with variant support for subagents - Implements variant support for subagents
• feat(task): pass variant to subagent so it inherits parent's thinking level #12567 - feat(task): pass variant to subagent so it inherits parent's thinking level - Related to variant inheritance in subagents
• feat(tui): preserve selected thinking effort per agent per model for primary agents #36708 - feat(tui): preserve selected thinking effort per agent per model for primary agents - TUI footer variant handling for main agents (similar pattern)

These are related to variant handling in subagents but don't appear to be addressing the specific feature of displaying model variant information in the subagent footer.

[dyool17]

This fix lands exactly what's missing in the subagent footer. I'm on opencode 1.18.15 (Windows): when I navigate into a subagent session I only see the agent name and context — the main prompt shows the variant, but the subagent footer drops it entirely. For reasoning models that matters: high vs xhigh can be a very different cost, and there's no way to tell what the subagent is actually running. This also ties into #38495 (model and token usage missing in subagent view) — it feels like one gap, not two. In the meantime I'm looking for a way to patch around this in my setup until it's merged. Happy to test once it's in a dev build.