feat(tui): match main model metadata in child sessions

[guanbear]

Issue for this PR

Closes #50795

Type of change

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

What does this PR do?

Opening a subagent's child session in v2 replaces the normal prompt with the Composer, so the usual model metadata row disappears. The child Composer now reuses the main prompt's PromptMetadataRow, reading the child's persisted model and resolving names from its location's catalog. It hides the row until the child has a model. The Subagents list is unchanged.

#38772 covers similar footer behavior on dev. #51232 targets the v2 Subagents list instead; this PR shows the model after opening the child, using the main prompt’s metadata row. I would appreciate guidance on the preferred UI placement and merge path.

How did you verify your code works?

• bun test test/cli/tui in packages/tui: 425 passed on the final head, updated to v2 at a604b7f7.
• bun typecheck in packages/tui; Prettier and oxlint on the changed files; repository lint and typecheck via the pre-push hook.
• Rendered the production child Composer with fixture sessions at 80 and 40 columns. These captures use fixture data, not a live delegated run.

Screenshots / recordings

Opened child session before, 80 columns:

Opened child session after, 80 columns:

Opened child session after, 40 columns:

Checklist

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

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[guanbear]

Update: I updated the branch against current v2 (a604b7f7; PR head 256e28cb2) and reused the main prompt's PromptMetadataRow in the opened child session. The Subagents list remains unchanged. I added focused tests for normal, missing-agent, and default-variant cases; the final v2 TUI suite passed (425 tests), as did package typecheck and the repository pre-push lint/typecheck. The PR now has refreshed 80- and 40-column captures of the production component with fixture data.

I also noted #51232, which proposes showing the model in the list, and #38772 on dev. This PR follows #50795's opened-child footer behavior. I would appreciate guidance on the preferred placement and merge path. Thank you.