feat(app): show running subagents and shells in the session header - #53247

Running subagents and background shells are now one click away in the session header, instead of two clicks deep in the session summary.

Before	After

Stop on hover	Click a shell

• Label: N working… when only subagents run, N running once a background shell runs too. It shimmers while work runs.
• Rows: agent-colored progress icon, agent name and task. A subagent row opens its session. A shell row scrolls to the call, expands the collapsed Used group and expands the call.
• Stop: a hover X, or Delete on a highlighted row. It uses the same SDK calls as the TUI composer tabs. The menu stays open, and the row leaves when the work ends.
• Scope: all running subagents, foreground or background. Background shells only, because the timeline already shows a foreground shell at the bottom.
• The session summary's N background tasks running row is removed, with its translations.

Inside a subagent

Swap between siblings	Details

• The menu lists the parent's running work, so sibling subagents are one click away. The current subagent is marked (aria-current).
• Shell rows and starting subagents open the parent session on that call: the route carries it, and the parent timeline scrolls to it and expands it instead of scrolling to its end.
• Session details now show for subagents. The workspace row is read-only and the "Move to worktree" hint is hidden: a subagent never moves on its own.

flowchart LR
  B[blocking subagents] --> M[SessionRunningMenu]
  T[background tasks] --> M
  M -->|subagent row| N[navigateToSession]
  M -->|shell row| R["virtualizer.revealPart: open group + call, scroll"]
  M -->|X / Delete| S{type}
  S -->|subagent| I["api.session.interrupt({ sessionID })"]
  S -->|shell| K["api.shell.remove({ id, location })"]

• Row icons use the theme's --v2-agent-<name>-solid tokens, so Explore and Build match the timeline cards; other agents stay neutral.
• The e2e mock server gains DELETE /api/shell/:id.