delegation/skills: cap browser-driving scoop fan-out and document bridge serialization

[catalan-adobe]

Context

Split out of the 2026-08-25 concurrent-scoop incident (browser-bridge hardening in PR #2411). The trigger for the whole incident was skill guidance prescribing unbounded browser fan-out: liftoff-demo Phase 3 and migrate-page Phase 3 instruct "Create AND feed ALL scoops in a SINGLE response", which put ~16 tabs / 8 concurrent drivers on one serialized browser bridge.

Note: liftoff-demo, migrate-page, and migrate-block are runtime-installed skills and do not live in this repo — this issue tracks the work and the repo-side pieces (delegation skill guidance, defaults), and points at the skill-side changes wherever those skills are authored.

Problem

• PR fix(playwright-cli): harden the browser bridge under concurrent scoop access #2411 makes contention visible (stderr note with lock wait + queue depth) and makes failures loud, but the bridge still serializes all drivers: 8-way fan-out inflates wall-clock ~2x with retry overhead even when nothing breaks.
• The skills also claim writablePaths "eliminates the approval stalls entirely", which the incident disproved (see the companion approval-system issue).

Suggested changes

Skill-side (where liftoff-demo / migrate-page / migrate-block are authored):

1. Cap concurrent browser-driving scoops at 3–4, or feed scoops in waves (semaphore pattern) instead of "ALL in a single response". Highest-value change.
2. Pass the source capture width to every scoop and have it assert the viewport before any comparison (resize → read back via eval innerWidth → abort on mismatch). PR fix(playwright-cli): harden the browser bridge under concurrent scoop access #2411 makes resize sticky per tab, but the assertion catches everything else.
3. Instruct scoops to treat the new note: browser bridge contended stderr signal as back-off guidance (wait/stagger) rather than re-running commands.
4. Pre-authorize /.migration/** alongside /.playwright/**, or write crops under the project path; drop the "eliminates approval stalls entirely" claim.

Repo-side candidates:

5. packages/vfs-root/workspace/skills/delegation/SKILL.md: add a note that scoops driving the shared browser serialize on one tab lock — recommend capping browser-heavy fan-out at 3–4 and citing the contended-bridge stderr note.
6. Cone-side liveness guidance: "files stable + scoops processing" is not completion; poll pending sudo requests when a fan-out goes quiet.

Acceptance

• Delegation skill documents the browser-bridge serialization constraint and the recommended fan-out cap.
• The externally-authored migration skills are updated per 1–4 (tracked here, implemented where they live).

[github-actions]

The code involved is a little too into performance art.

[github-actions]

^{🎫 Backlog Dispatcher}

Not dispatching this one. Most of the work lives in externally-authored runtime skills outside this repo and the remaining repo-side pieces are a concurrency/fan-out design call (browser-bridge serialization, approval-system interplay) that a human should scope, not a localised fix.

Nothing was changed and no pull request was opened. This decision is made once — remove the backlog-skipped label to put the issue back in the queue.

[augmentcode]

^{Backlog Dispatcher🎫}

Skipped by Backlog Dispatcher. Reason: Cross-repository change requiring runtime-skill ownership and broader coordination.

[augmentcode]

^{Backlog Dispatcher🎫}

Skipped by Backlog Dispatcher. Reason: Work spans runtime-installed skills outside this repository plus multiple repo-side policy changes.