feat(bin): gstack-design-detect wrapper + design_detector config key

bin/gstack-design-detect.ts finds and runs an impeccable engine the user
installed; it never installs, downloads, or executes anything that could
download. `probe` reads only: config (design_detector off → DISABLED),
IMPECCABLE_BIN (absolute, realpath outside the repo and cwd), a PATH walk
(absolute entries outside the repo; a #! shim counts as launcher-present,
never READY), the ~/.impeccable/bin/<newest semver>/ cache, and the engine
installed beside a skill launcher (scripts/bin/<os>-<arch>/impeccable, the
layout a real install produced). It reports IMPECCABLE_SKILL, host-aware
IMPECCABLE_HOOK (+ HOOK_OTHER), the ignore lists from .impeccable/config*.json,
IMPECCABLE_ENGINE_UNTESTED for versions outside the fixture set, and a hint
only when a launcher exists without its engine. `scan` re-probes, refuses
URLs and anything outside the repo root or the design-report allow-list
(realpath, so symlinks cannot escape), derives `--changed <base>` targets
NUL-safely through git and lib/frontend-scope.ts, batches 100 absolute paths
per engine call with stdin ignored, a SIGKILL timeout, a 50 MB stdout cap, and
sanitized length-capped fields, then prints one normalized JSON document
(--format gstack) or the engine's bytes (--format raw); DETECT_TOP (fenced as
untrusted content), DETECT_SUMMARY, and DETECT_EXIT go to stderr; exit code
passes through with 1 over 2 over 0; exit 3 is a gstack bug. `rules` prints
the mapped set. Every run appends a content-free line to the local analytics
file.

lib/design-detect-contract.ts owns every sentinel string, the limits, and the
normalized-finding shape (pure module); test/design-detect-contract.test.ts
asserts every sentinel-shaped token the agent can read exists there.
lib/frontend-scope.ts mirrors gstack-diff-scope's frontend arm, pinned by a
parity test that runs the bash script. bin/gstack-config gains
design_detector (auto | off, default auto, invalid values rejected with the
file unchanged). test/fixtures/fake-impeccable.ts is the env-driven engine
stand-in; test/gstack-design-detect.test.ts covers READY/NOT_CACHED/
NOT_AVAILABLE/DISABLED, env trust (.env never loaded, in-repo IMPECCABLE_BIN
ignored), newest-semver cache, hook and ignore detection, refusals, exit
passthrough, raw byte-identity, normalization, the display cap, timeout,
parse errors, diagnostics, --changed, and analytics. The egress scanner test
records the wrapper as a documented non-sink.

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

bin/gstack-config

@@ -122,6 +122,13 @@ 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: auto     # Deterministic design pre-pass through a user-installed
+#                           #   impeccable engine (/design-review, /review, /ship,
+#                           #   /design-html). auto = use the engine when the probe
+#                           #   finds one (gstack never installs or downloads it);
+#                           #   off = no probe, no scan, no hint, no /impeccable
+#                           #   handoff lines. An invalid value is REJECTED (existing
+#                           #   value preserved) so a typo cannot silently disable it.
 # gstack_contributor: false # true = file field reports when gstack misbehaves
 # skip_eng_review: false    # true = skip eng review gate in /ship (not recommended)
 #
@@ -149,6 +156,7 @@ lookup_default() {
     checkpoint_push) echo "false" ;;
     explain_level) echo "default" ;;
     codex_reviews) echo "enabled" ;;
+    design_detector) echo "auto" ;; # auto | off — impeccable engine pre-pass in the design skills
     gstack_contributor) echo "false" ;;
     skip_eng_review) echo "false" ;;
     workspace_root) echo "$HOME/conductor/workspaces" ;;
@@ -416,6 +424,12 @@ case "${1:-}" in
       echo "Error: codex_reviews '$VALUE' not recognized. Valid values: enabled, disabled. Existing value left unchanged." >&2
       exit 1
     fi
+    # design_detector gates a third-party binary the user installed. Reject a typo
+    # rather than coerce it: "of" must not silently re-enable or disable the scan.
+    if [ "$KEY" = "design_detector" ] && [ "$VALUE" != "auto" ] && [ "$VALUE" != "off" ]; then
+      echo "Error: design_detector '$VALUE' not recognized. Valid values: auto, off. Existing value left unchanged." >&2
+      exit 1
+    fi
     # cross_project_learnings: empty get is the first-run prompt sentinel.
     # Skills enable only on the literal "true". A typo must not persist — that
     # keeps the feature off and suppresses the prompt. Reject, like
@@ -455,7 +469,7 @@ case "${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; do
+               timeline_stop_hook design_detector; do
       VALUE=$(read_config_value "$KEY" || true)
       SOURCE="default"
       if [ -n "$VALUE" ]; then
@@ -472,7 +486,7 @@ case "${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; do
+               timeline_stop_hook design_detector; do
       printf '  %-24s %s\n' "$KEY:" "$(lookup_default "$KEY")"
     done
     ;;