feat(tui): show subagent status in sidebar

[shirafukayayoi]

Issue for this PR

No issue. Related: #4865 and #25712.

Type of change

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

What does this PR do?

Adds a small built-in TUI sidebar section for child sessions started by subagents.

The panel reads the existing synced session list and session status map, filters children by the current session ID, and shows a compact count plus up to five child session titles. Running children use the warning color and completed children use muted text.

This is intentionally smaller than #4865: it does not add navigation or keybindings. It is also separate from #25712, which focuses on subagent cost rollups.

How did you verify your code works?

• bun run --cwd packages/tui typecheck

I also tested the same sidebar behavior locally in a custom build before reducing this PR to the minimal upstream diff.

Note: the repository pre-push hook currently runs the full workspace typecheck, which failed on an existing packages/app/src/custom-elements.d.ts parse error unrelated to this TUI change. The package-level TUI typecheck above passes.

Screenshots / recordings

Not included. The panel renders only when the current session has child sessions, with text like Subagents 2 running, 1 done and one line per child session.

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, here are the related PRs that might be relevant to this feature:

Most Related:

• feat(tui): show subagent cost rollup in sidebar and task history #25712 - feat(tui): show subagent cost rollup in sidebar and task history - The PR description explicitly mentions this is separate from feat(tui): show subagent cost rollup in sidebar and task history #25712, which focuses on subagent cost rollups. This is a closely related but distinct feature in the same area.

Also Related:

• feat: add subagents sidebar with clickable navigation and parent keybind #4865 - Referenced in the PR description as related to subagent navigation (though not found in search results, it was mentioned as context)
• fix(tui): restore subagent navigation #35762 - fix(tui): restore subagent navigation - Related to subagent display in TUI
• feat(opencode): add agent teams and nested subagent delegation #33144 - feat(opencode): add agent teams and nested subagent delegation - Related to subagent/nested agent functionality
• feat: add team TUI integration — sidebar, header, sync, and team dialog #12732 - feat: add team TUI integration — sidebar, header, sync, and team dialog - Related to TUI sidebar work

These are all related features in the subagent/TUI sidebar ecosystem, but #36042 appears to be the only PR implementing this specific feature (showing subagent status counts and titles in the sidebar). The PR is appropriately scoped as a minimal feature separate from the cost rollup work (#25712).