feat(design-detect): consent-gated engine install, checksum-pinned and receipted

`gstack-design-detect.ts install` is the one download gstack makes, and only
after a design skill's one-time question got a yes. It fetches the engine
version gstack has tested (0.1.3) for this platform from impeccable's own
GitHub release, verifies it against the checksum pinned in
lib/design-detect-contract.ts (all five platforms, captured from the
release's .sha256 sidecars; linux-x64 equals the fixture engine), writes an
egress receipt before the fetch and refuses to download when the receipt
cannot be written (fail-closed; the sink is registered in the wiring test's
polarity table), caps the download at 32 MB, streams with the cap enforced,
writes the file only after the hash matches, and places it under
~/.impeccable/bin/<version>/ (a trusted IMPECCABLE_HOME is honored; never
inside a project). No skill, no hook, no launcher, no npx. --sha256 accepts a
sidecar checksum for a version gstack has not pinned; --base allows a mirror
(https, or http on loopback for tests). After a successful install the probe
runs and its lines follow, so the skill sees READY at once.

The probe ends with DESIGN_DETECTOR_INSTALL_OFFER (version, platform, bytes,
destination) whenever it found no engine and the user has not answered the
question; once design_detector_install_prompted is true it prints neither the
offer nor the NOT_CACHED hint, which used to repeat on every run. The hint's
npx wording is corrected: `npx impeccable detect --help` caches the engine
for npx only, not where the probe looks.

gstack-config gains design_detector_install_prompted (true|false, typo
rejected, enumerated in list and defaults). Tests: a loopback mirror (async
spawn, so the in-process server can answer) covers install, re-install as a
verified no-op, checksum mismatch, 404, unpinned version, non-https base,
design_detector off, and IMPECCABLE_HOME inside the repo; the offer and the
silenced hint; pin completeness per platform.

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

bin/gstack-config

@@ -122,6 +122,10 @@ CONFIG_HEADER='# gstack configuration — edit freely, changes take effect on ne
 #                           #   /document-release skip the outside-voice step entirely.
 #                           #   An invalid value is REJECTED (existing value preserved) so
 #                           #   a typo cannot silently turn paid Codex calls on or off.
+# design_detector_install_prompted: false
+#                           #   true once you answered the one-time offer from the
+#                           #   design skills to download the impeccable engine with
+#                           #   "never ask again"; flip back to false to be asked again.
 # design_detector: auto     # Deterministic design pre-pass through a user-installed
 #                           #   impeccable engine (/design-review, /review, /ship,
 #                           #   /design-html). auto = use the engine when the probe
@@ -157,6 +161,7 @@ lookup_default() {
     explain_level) echo "default" ;;
     codex_reviews) echo "enabled" ;;
     design_detector) echo "auto" ;; # auto | off — impeccable engine pre-pass in the design skills
+    design_detector_install_prompted) echo "false" ;; # true once the user answered the one-time engine install offer with "never ask again"
     gstack_contributor) echo "false" ;;
     skip_eng_review) echo "false" ;;
     workspace_root) echo "$HOME/conductor/workspaces" ;;
@@ -424,6 +429,13 @@ case "${1:-}" in
       echo "Error: codex_reviews '$VALUE' not recognized. Valid values: enabled, disabled. Existing value left unchanged." >&2
       exit 1
     fi
+    # design_detector_install_prompted records "never ask again" for the one-time
+    # engine install offer. Rejecting a typo keeps the offer from silently coming
+    # back (or never coming back) because of a mistyped value.
+    if [ "$KEY" = "design_detector_install_prompted" ] && [ "$VALUE" != "true" ] && [ "$VALUE" != "false" ]; then
+      echo "Error: design_detector_install_prompted '$VALUE' not recognized. Valid values: true, false. Existing value left unchanged." >&2
+      exit 1
+    fi
     # design_detector gates a third-party binary the user installed. Reject a typo
     # rather than coerce it: "of" must not silently re-enable or disable the scan.
     if [ "$KEY" = "design_detector" ] && [ "$VALUE" != "auto" ] && [ "$VALUE" != "off" ]; then
@@ -469,7 +481,7 @@ case "${1:-}" in
                skill_prefix checkpoint_mode checkpoint_push explain_level \
                codex_reviews gstack_contributor skip_eng_review workspace_root \
                artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks \
-               timeline_stop_hook design_detector; do
+               timeline_stop_hook design_detector design_detector_install_prompted; do
       VALUE=$(read_config_value "$KEY" || true)
       SOURCE="default"
       if [ -n "$VALUE" ]; then
@@ -486,7 +498,7 @@ case "${1:-}" in
                skill_prefix checkpoint_mode checkpoint_push explain_level \
                codex_reviews gstack_contributor skip_eng_review workspace_root \
                artifacts_sync_mode artifacts_sync_mode_prompted plan_tune_hooks \
-               timeline_stop_hook design_detector; do
+               timeline_stop_hook design_detector design_detector_install_prompted; do
       printf '  %-24s %s\n' "$KEY:" "$(lookup_default "$KEY")"
     done
     ;;