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>

CHANGELOG.md

@@ -5,7 +5,7 @@
 **gstack's design skills now start with 61 deterministic anti-pattern checks, in your voice, then spend their judgment where a detector cannot.**
 **DESIGN.md is written in the open format that impeccable and Google Stitch read, and the AI-slop list lives in one typed catalog instead of four drifting copies.**
 
-If you have [impeccable](https://impeccable.style) installed, `/design-review`, `/design-html`, `/review`, and `/ship` run its engine first and hand you its findings as `FINDING-NNN [rule-id]` rows with file:line and a handoff (`/impeccable typeset`, `layout`, `colorize`, ...) before the LLM pass reads a page. gstack never installs, downloads, or runs anything that could download: the probe only reads files, the engine runs only from an install under your home directory, and a checked-out repository can never make gstack execute one of its own files. Without impeccable nothing changes and nothing nags; `gstack-config set design_detector off` silences every trace.
+If you have [impeccable](https://impeccable.style) installed, `/design-review`, `/design-html`, `/review`, and `/ship` run its engine first and hand you its findings as `FINDING-NNN [rule-id]` rows with file:line and a handoff (`/impeccable typeset`, `layout`, `colorize`, ...) before the LLM pass reads a page. gstack never runs impeccable's installer or launcher. The first time a design skill finds no engine it asks once: download the 16 MB engine binary from impeccable's own GitHub release into `~/.impeccable`, checked against a checksum gstack pins and recorded in your egress ledger first, or not. Say no and nothing changes and nothing nags; `gstack-config set design_detector off` silences every trace. The probe only reads files, the engine runs only from under your home directory, and a checked-out repository can never make gstack execute one of its own files.
 
 On a live URL, `/design-review` scans the rendered page: the DOM is dumped with linked stylesheets inlined and scripts, input values, handlers, and query strings stripped, redaction-checked, kept owner-only, and the engine reads that, so Rule 4 holds on a deployed site. `/design-consultation` writes the open DESIGN.md spec (tokens in front matter, eight canonical sections, your Motion and Decisions Log kept). An existing file is converted only when you say so, once, and the answer is recorded in the file.
 
@@ -15,7 +15,7 @@ Source: `git diff origin/main --shortstat`, `lib/design-catalog.ts` (`bun -e` ov
 
 | Metric | Before | After | Δ |
 |---|---|---|---|
-| Deterministic checks in a design review | 0 | 61 rules (impeccable's engine, when you installed it) | evidence before judgment |
+| Deterministic checks in a design review | 0 | 61 rules (impeccable's engine, one consented download away) | evidence before judgment |
 | Places the AI-slop vocabulary lived | 4 files, drifting | 1 typed catalog, 86 entries (57 slop, 29 quality) | `review/design-checklist.md` is generated from it |
 | Skills that run the detector | 0 | 4 (`/design-review`, `/design-html`, `/review`, `/ship`) | same rule ids everywhere |
 | What a scan can read | source files | source files or the rendered DOM of a live URL | works on deployed sites |
@@ -24,12 +24,13 @@ Source: `git diff origin/main --shortstat`, `lib/design-catalog.ts` (`bun -e` ov
 
 The row you feel is the first one: a purple gradient, a kicker above a heading, a nested card, or `Inter` as the display face is a machine finding with a file:line before anyone forms an opinion, so the review's judgment goes to hierarchy, trust, and copy.
 
-What this means for anyone shipping a UI with gstack: install impeccable once in your own shell if you want the engine, then run `/design-review` as before. The mechanical rows arrive first, tagged, deduplicated against the checklist, and calibrated against your DESIGN.md tokens. Never install it and every design skill behaves as in v1.81, with sharper doctrine.
+What this means for anyone shipping a UI with gstack: say yes once when a design skill offers the engine (or install impeccable yourself), then run `/design-review` as before. The mechanical rows arrive first, tagged, deduplicated against the checklist, and calibrated against your DESIGN.md tokens. Never install it and every design skill behaves as in v1.81, with sharper doctrine.
 
 ### Itemized changes
 
 #### Added
 - **Design detector** (`bin/gstack-design-detect.ts`, config key `design_detector: auto|off`): `probe` reports `IMPECCABLE_READY | NOT_CACHED | NOT_AVAILABLE | DISABLED` plus skill, hook, and config-ignore lines without executing anything; `scan` runs the user-installed engine over repository files, changed frontend files (`--changed <base>`, NUL-safe, batched), or DOM dumps under `~/.gstack/projects/<slug>/designs/`, and prints one normalized JSON document plus a fenced, untrusted `DETECT_TOP` block grouped by rule; `rules` lists the mapping. Only an install under HOME (cache, env override, PATH, or the skill's sibling engine) whose real file is named `impeccable` is ever READY; URLs are refused; the engine sees a minimal environment; per-batch and whole-scan timeouts; findings capped and every field sanitized; exit 3 marks a gstack bug.
+- **Consent-gated engine install** (`gstack-design-detect.ts install`): when the probe finds no engine, the design skills ask one question, once, in interactive sessions only (never in spawned or headless runs). Yes downloads the engine version gstack has tested for your platform from github.com/pbakaus/impeccable's releases, verifies it against the checksum pinned in gstack for all five platforms, writes an egress receipt before the fetch (fail-closed), and places the one file under `~/.impeccable/bin/<version>/`: no impeccable skill, no editor hook, no launcher. "Never ask again" is remembered in `design_detector_install_prompted`; "turn it off" is `design_detector off`. gstack never runs `npx impeccable`.
 - **Phase 0 in `/design-review`**: source mode on a feature branch, DOM mode on any URL (Aside first, the bundled browser otherwise) through one shared dump script, `lib/dom-dump.js`; `design-baseline.json` gains per-page detector counts with id-level deltas and an `engine changed` caveat; Phase 10 reports `Detector: N → M`; deferred findings end with the `/impeccable <cmd>` handoff when the skill is installed.
 - **`/review` and `/ship`** design pass: the mechanical scan runs first, its rows bucket by tier (auto-fix, ask, possible), and a detector hit at the same file:line as a checklist hit is one row. **`/design-html`**: a bounded slop gate before screenshots, one fix pass, then accepted-with-reason rows.
 - **Typed design catalog** (`lib/design-catalog.ts`): 86 entries with rule ids, impact, tier, confidence, detection method, handoff, fonts, and the ten `mockupNever` patterns the image-generation prompt now refuses by default. `review/design-checklist.md` is generated from it by `bun run gen:skill-docs`.