feat(design): ask once before downloading impeccable's engine

When the probe prints DESIGN_DETECTOR_INSTALL_OFFER the design skills ask the
user one AskUserQuestion, in interactive sessions only (spawned or headless
runs never install and never ask; Conductor gets the prose brief), before
any other step: install the engine now, not now, never ask again
(design_detector_install_prompted), or turn the detector off. A yes runs the
receipted, checksum-pinned install and the skill continues with a READY probe.
The brief says what impeccable is, what the one file is, where it goes, how
it is verified and logged, and that no skill or hook comes with it; users who
want the /impeccable skill run npx impeccable install themselves.

design-review carries the brief inline (it is not carved). design-html keeps
its skeleton small: the probe block points at a new read-on-demand section,
sections/detector-install-offer.md, registered in its manifest and carve
guard; its skeleton ceiling is re-measured (55,262) and its eager ceiling
set to the measured 13,767. The review and ship passes state that they never
offer an install. NOTICE.md, README, docs/skills.md, the interop design doc,
and the CHANGELOG describe the new posture: gstack still never runs
impeccable's installer or launcher; the one download is consented, pinned,
and receipted. Ship goldens refreshed for the review-pass wording.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -319,7 +319,7 @@ gstack works well with one sprint. It gets interesting with ten running at once.
 
 **`/design-shotgun` is how you explore.** You describe what you want. It generates 4-6 AI mockup variants using GPT Image. Then it opens a comparison board in your browser with all variants side by side. You pick favorites, leave feedback ("more whitespace", "bolder headline", "lose the gradient"), and it generates a new round. Repeat until you love something. Taste memory kicks in after a few rounds so it starts biasing toward what you actually like. No more describing your vision in words and hoping the AI gets it. You see options, pick the good ones, and iterate visually.
 
-**Works with impeccable.** If you use [impeccable](https://impeccable.style) too, gstack does not fight it. gstack runs impeccable's deterministic engine as a pre-pass in `/design-review`, `/review`, `/ship`, and `/design-html` when you have it installed (gstack never installs, downloads, or runs anything that could download it), speaks the same 61 rule ids in its own voice, reads `PRODUCT.md`, writes `DESIGN.md` in the open DESIGN.md format both tools read, and hands deferred findings to `/impeccable <command>`. Without impeccable, nothing changes: no nag, no missing step. `gstack-config set design_detector off` turns the pre-pass off. Attribution for the material gstack derived from impeccable and the DESIGN.md spec is in `NOTICE.md`.
+**Works with impeccable.** If you use [impeccable](https://impeccable.style) too, gstack does not fight it. gstack runs impeccable's deterministic engine as a pre-pass in `/design-review`, `/review`, `/ship`, and `/design-html` when you have it installed (gstack never runs impeccable's installer or launcher; the first time a design skill finds no engine it asks once whether to download the engine binary, checksum-pinned and logged, into `~/.impeccable`, and remembers your answer), speaks the same 61 rule ids in its own voice, reads `PRODUCT.md`, writes `DESIGN.md` in the open DESIGN.md format both tools read, and hands deferred findings to `/impeccable <command>`. Say no and nothing changes: no nag, no missing step. `gstack-config set design_detector off` turns the pre-pass off. Attribution for the material gstack derived from impeccable and the DESIGN.md spec is in `NOTICE.md`.
 
 **`/design-html` makes it real.** Take that approved mockup (from `/design-shotgun`, a CEO plan, a design review, or just a description) and turn it into production-quality HTML/CSS. Not the kind of AI HTML that looks fine at one viewport width and breaks everywhere else. This uses Pretext for computed text layout: text actually reflows on resize, heights adjust to content, layouts are dynamic. 30KB overhead, zero dependencies. It detects your framework (React, Svelte, Vue) and outputs the right format. Smart API routing picks different Pretext patterns depending on whether it's a landing page, dashboard, form, or card layout. The output is something you'd actually ship, not a demo.