Parity with the classic CLI status bar's ⛓ indicator (PR #51441). The
Ink TUI status bar now shows ⛓ N for live background/async subagents
(delegate_task batches + background single delegations).
- tui_gateway/server.py: _get_usage() embeds active_subagents from
tools.async_delegation.active_count() — the same registry the CLI
reads — onto the existing per-update usage payload, guarded so a
raising active_count() leaves the field off without breaking usage.
- ui-tui appChrome: new 'subagents' status segment (breakpoint w>=92,
slots between bg and cost in the shed-order), renders ⛓ N from
usage.active_subagents.
- Usage / SessionUsageResponse types gain active_subagents?.
Distinct from the turn-scoped SpawnHud / /agents overlay, which mirror
live in-turn subagent.* events; this is the persistent registry count.
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