feat: inspect delegated work in the Subagents panel #919

Parent

#916

What to build

Let developers select any active or finished roster row and inspect that sub-agent in the same right panel. The detail view must keep the assignment, lifecycle, nested activity, final output, bounded-output evidence, and safely attributed file changes together without inventing missing provider data.

Use the focused-detail reference artifact from #916. Keep the reading surface tonal and restrained, with a clear back action and no decorative nested guide rails.

Acceptance criteria

• Selecting an Active or Finished row opens the matching detail in the same panel.
• Back returns to the prior horizontal tab and preserves its list position and selection.
• The detail header shows agent identity, explicit status, and elapsed or terminal duration.
• The delegated task remains visible above the result and activity.
• Nested tool calls and nested sub-agents stay attributed to the correct parent through indentation and breadcrumb context.
• Completed output renders through the settled Markdown treatment.
• Truncated output retains the existing artifact notice and access to the full result.
• File changes appear only when existing data attributes them safely to the selected sub-agent.
• Long child histories remain bounded and expose an explicit expansion control.
• Keyboard and screen-reader users can open, inspect, navigate nested work, and return.
• Focused tests cover active and terminal details, nested attribution, back navigation, Markdown output, truncation, and bounded histories.
• Live verification inspects running, completed, and errored agents at 440px and 384px with reduced motion enabled once.
• The repository-wide verification gate passes.

Blocked by

• feat: retain finished subagents across hydration #918