feat(tui): show child model beside subagent status

[opencode-agent]

Summary

• Show each child session’s selected model in the Subagents tab, even when it matches the parent. Resolve the child-location catalog name; fall back to provider/model and show its variant when present. If the child has no selected model, do not guess.
• Keep the model aligned before the status with a · separator, matching the B layout discussed for this view. Truncate task and model independently at narrow terminal widths.

Validation

• bun test test/cli/tui/composer-keymap.test.tsx (8 passed)
• GOMAXPROCS=4 bun typecheck in packages/tui
• bun run lint and git diff --check
• Full bun run check could not finish on this Host: system Node 18 fails the posts check, and retrying with Node 24 aborted in the unrelated www typecheck under shared resource limits.

Related: #50795. Alternative to #50798, which implements overlapping model visibility without this model-to-status separator; please choose one implementation rather than merging both.

Requested by: @rekram1-node (Aiden via Slack)