TUI: make sequential / multiple delegations legible

[aheritier]

Part of #3104

When several transfer_task calls happen in one turn (they run sequentially), the transcript is one undifferentiated stream and the sidebar shows only a single working agent, so the delegation structure is lost.

Proposed changes

1. Transcript delegation brackets. On child stream start/stop, render theme-styled delimiter lines, e.g. ── root → librarian ───── on entry and a matching completion marker on exit. Drive these from AgentSwitchingEvent (carries FromAgent/ToAgent) and by handling SubSessionCompletedEvent in pkg/tui/page/chat/runtime_events.go (currently unhandled). Reconcile with messages.needsSeparator (which already special-cases transfer_task).
2. Sidebar delegation breadcrumb + background count. In pkg/tui/components/sidebar/sidebar.go, extend the single workingAgent notion to show a short chain (e.g. root ⏵ librarian) when streamDepth > 1, plus a count badge when ≥1 background agents are active. Reuse AgentAccentStyleFor for per-agent color.

Acceptance criteria

• Sequential delegations render as distinct, labeled sections.
• Returning control to the parent is visually marked.
• The sidebar shows the active delegation chain, not just the deepest single agent.

Validation

• task build && task test && task lint green.
• Test that SubSessionCompletedEvent produces the completion delimiter and keeps streamDepth balanced.
• Integrate new transcript lines with existing separator/scroll/selection logic (extend pkg/tui/components/messages tests).