feat: live "dispatch deck" — footer status for in-flight subagents

[randomm]

Problem

When PM dispatches one or more subagents (via dispatch_specialist / dispatch_parallel, or dispatch_lens_review), the user sees nothing in the parent UI while children run. Long children mean minutes of silent spinner — see screenshot context in [this session's design discussion].

We already compute the data we'd need to show: extension/src/progress.ts:17-40 defines RunningState (role, turns, lastToolName, lastText, usage, elapsed) and spawn.ts updates it on every message_end event via ingestEvent(). After the async pivot (#19/#20), the onProgress callback that used to push this into Pi's onUpdate tool-block is no longer wired — the state is computed and discarded.

Pi's extension API exposes the right primitive: ctx.ui.setStatus(id, text) — a persistent footer status entry, addressable by id, theme-aware. Reference: extension/node_modules/@earendil-works/pi-coding-agent/examples/extensions/status-line.ts:15-31.

Design

A single "dispatch deck" status entry, rendered as multi-line text we own end-to-end. One key ("ensemble:deck") — we control the layout, so ordering and overflow are ours.

Per-row format

⏳ developer · 2m14s · bash (#7) — Running test suite in worktree-A

• Icon: ⏳ running, ✓ done, ✗ failed
• Role label (with optional tag for lens-review children, e.g. code-review[security])
• Elapsed (e.g. 47s, 2m14s)
• Last tool name + tool-use count
• Truncated lastText (≤60 chars, newlines stripped)

Ordering: insertion order, stable

First-dispatched-first-displayed. Once a row exists, it stays at that index until removed. Backed by Map<jobId, RunningState> iterated in insertion order (JS Maps preserve this natively).

Linger: 0s on completion

On child completion (success OR failure), the row drops immediately. Persistent record lives in lifecycle scrollback entries (separate issue) and in PM's reaction text.

Overflow: 4 rows + summary

⏳ ensemble dispatch · 5 running · 1 done · 1m04s elapsed
 ↳ developer · 1m04s · bash
 ↳ explore   · 1m04s · grep
 ↳ ops       · 47s   · gh
 ↳ adversarial · 23s · diff
 (+1 more — use dispatch_status for full list)

Picks the 4 most-recently-active.

Sources

All three feed the same deck via existing onProgress(state) emissions:

• dispatch_specialist (single async) — extension/src/spawn.ts:328,398
• dispatch_parallel (multi async) — same onProgress path
• dispatch_lens_review (6 sync lens children) — extension/src/lens-review.ts:247

Opt-out

PI_ENSEMBLE_QUIET_STATUS=1 disables the deck entirely.

Acceptance criteria

• New module extension/src/dispatch-deck.ts (~120 LoC); exports update(jobId, state), clear(jobId); captures ExtensionContext via session_start listener
• spawn.ts and lens-review.ts both feed the deck via their existing onProgress callbacks (no new event plumbing — just route to the new module)
• Row ordering is stable across updates (insertion order)
• Multi-line render fits Pi's footer (no Pi-side wrapping; we truncate per row)
• Overflow at >4 children renders summary header + 4 detail rows + "+N more"
• Row drops at 0s on child completion (no linger)
• PI_ENSEMBLE_QUIET_STATUS=1 opt-out honoured
• Offline smoke test (smoke-tests/test-dispatch-deck.ts) covers: render single child, render parallel batch, ordering stability under interleaved updates, overflow rendering, opt-out env var
• Visually verified via live smoke (extend test-progress-live.ts or new test-dispatch-deck-live.ts) — footer renders during real parallel dispatch
• All existing offline smoke tests still pass
• tsc + biome clean

Out of scope (other issues)

• Lifecycle scrollback entries (separate issue) — durable record of dispatch/complete/fail events
• dispatch_peek tool (research: PM peek-inside-running-subagent (no transcript bloat) #21) — PM-callable query, builds on the same state map
• Drop inline onUpdate from dispatch_lens_review (separate issue) — once the deck is the single live surface

References

• Live progress data structure: extension/src/progress.ts:17-40
• Current onProgress emit sites: extension/src/spawn.ts:328,398, extension/src/lens-review.ts:247
• Pi UI primitive: ctx.ui.setStatus() — see examples/extensions/status-line.ts:15-31
• Pi ExtensionAPI definition: extension/node_modules/@earendil-works/pi-coding-agent/dist/core/extensions/types.d.ts:780+
• Async pivot: feat(dispatch): pivot to all-async subagent dispatch via push-callback #19, feat: async dispatch pivot + adapter architecture + PM safety + branch hygiene #20
• Existing introspection surface: dispatch_status in extension/src/dispatch-status.ts