fix(skills/update-stack): switch drift gate scan to git diff (catches missing-locally case)

.claude/skills/update-stack/SKILL.md

@@ -101,17 +101,21 @@ git fetch devkit-vue master --quiet
 
 drift_found=0
 while IFS= read -r f; do
-  upstream_blob=$(git ls-tree devkit-vue/master -- "$f" 2>/dev/null | awk '{print $3}')
-  [ -z "$upstream_blob" ] && continue  # downstream-only file — skip
-  local_blob=$(git rev-parse "HEAD:$f" 2>/dev/null)
-  if [ "$upstream_blob" != "$local_blob" ]; then
-    echo "BLOCK: drift on shared stack file: $f"
+  # Both sides exist + differ → drift
+  if git cat-file -e "devkit-vue/master:$f" 2>/dev/null && git cat-file -e "HEAD:$f" 2>/dev/null; then
+    echo "BLOCK: drift on shared file: $f"
     echo "  Fix A — revert to upstream: git checkout devkit-vue/master -- $f"
     echo "  Fix B — promote upstream:   open a devkit PR with the change, merge, /update-stack here"
     echo "  Fix C — relocate:           move logic to a downstream-only module or src/config/defaults/<project>.config.js"
     drift_found=1
+  # Upstream has it, local doesn't → missing-locally
+  elif git cat-file -e "devkit-vue/master:$f" 2>/dev/null; then
+    echo "BLOCK: missing locally (was on upstream): $f"
+    echo "  Fix — restore: git checkout devkit-vue/master -- $f"
+    drift_found=1
   fi
-done < <(git ls-files src/modules src/lib src/config 2>/dev/null \
+  # Downstream-only file (not in upstream) → skip silently
+done < <(git diff --name-only devkit-vue/master HEAD -- src/modules src/lib src/config 2>/dev/null \
   | grep -vE "/(tests|__tests__)/" | grep -vE "\.(test|spec)\.(js|jsx|ts|tsx|vue)$")
 
 [ "$drift_found" -eq 1 ] && exit 1
@@ -120,7 +124,7 @@ echo "3ter: no drift — OK"
 
 **Rules:**
 - Block on ANY shared-file divergence. No "declare and skip" path — the `DOWNSTREAM_PATCHES.md` ledger model was abandoned 2026-06-02 (memory `feedback_no_dev_in_shared_modules`).
-- Scan covers the full stack tree (`src/modules`, `src/lib`, `src/config`) — auto-discovers every shared module. Per-file `git ls-tree` on upstream filters downstream-only files.
+- Scan source is `git diff --name-only devkit-vue/master HEAD` (bidirectional) — catches both files that differ AND files present upstream but missing locally (deleted downstream). The previous `git ls-files` approach only saw locally-present files.
 - Test files (paths containing `/tests/` or `/__tests__/`, or filenames ending `.test.{js,jsx,ts,tsx,vue}` / `.spec.{js,jsx,ts,tsx,vue}`) are excluded — downstream test adaptations are acceptable.
 - E2E helpers under `src/lib/helpers/e2e/` ARE scanned — they are stack-managed. Downstream modification triggers BLOCK; use Fix B (promote upstream) if a downstream needs e2e helper changes.
 - `src/modules/app/app.router.js` historically diverged on every downstream (downstream routes). Under no-ledger this must be refactored: `app.router.js` does NOT currently expose an extension hook — the refactor needs to add one (e.g. a `registerDownstreamRoutes()` call sourced from a downstream-only module like `src/modules/{project}/{project}.router.js`), then keep `app.router.js` stack-iso. Until that refactor lands, this gate will BLOCK on every Vue downstream `/update-stack`.