The classic prompt_toolkit status bar already shows two background
indicators: ▶ N (/background agent threads) and ⚙ N (shell processes
spawned by terminal(background=true)). Background/async subagents
(delegate_task batches and background single delegations) had no
indicator despite being long-running work the user should be able to
see at a glance.
Add a third indicator ⛓ N sourced from
tools.async_delegation.active_count() — the count of delegations still
in the 'running' state. Renders in the plain-text builder and the
styled-fragment builder across the same width tiers as the other two
(omitted on the narrow <52 tier), guarded so a raising active_count()
leaves the snapshot at 0.
The CLI status bar tracked /background agent tasks (▶ N) but not shell
processes spawned via terminal(background=true). Both kinds of work can
run concurrently and a user has no in-bar signal for shell processes.
Add an independent indicator (⚙ N) sourced from
tools.process_registry.process_registry._running. The two indicators
render side-by-side when both are active (▶ 1 │ ⚙ 2), hidden when their
count is zero. Renders at all four status-bar tiers (text fallback +
prompt_toolkit fragments, narrow + wide widths). The narrow <52 tier
still drops both for space — unchanged.
New ProcessRegistry.count_running() returns len(_running) without
acquiring _lock; CPython dict len is atomic and we're polling on every
status-bar tick, so lock-free is the right tradeoff.
Surface live background-task count in the prompt_toolkit status bar so users
can see at a glance that a /background task exists and is running — no need
to ask the agent about it (the agent has no visibility into bg sessions by
design).
- _get_status_bar_snapshot now reports active_background_tasks from len()
of the live _background_tasks dict (entries are removed in the task
thread's finally block, so this reflects truly-running tasks)
- Indicator shown only on medium (<76) and wide (>=76) tiers; narrow (<52)
stays minimal since it's already cramped
- No invalidate plumbing needed: status bar fragments are pulled via lambda
on every redraw, and the bg thread already calls _app.invalidate() on exit
Refs #8568
