feat(skills): vendor agent-skills trio at a pinned commit, emil precedent

observability-and-instrumentation, deprecation-and-migration,
ci-cd-and-automation from addyosmani/agent-skills 2686b620, curl'd into
skills-external/<name>/ by install-plugins.sh Step 8e (tmp+mv), refreshed
by update-all.sh 7.3, symlinked by link.sh, registered in toggle-external,
profile.sh and the full/backend/dev profiles. Pin read from the lock via
argv, never hardcoded. Case 2 of the 6-repo review, BDR-102.

lib/tests/toggle-external-repo-resolution.test.sh

@@ -14,6 +14,7 @@ check() { if [ "$2" = "$3" ]; then pass=$((pass+1)); else fail=$((fail+1));
 
 SANDBOX="$(mktemp -d)"
 mkdir -p "$SANDBOX/repo/lib" "$SANDBOX/repo/skills-external/emil-design-eng" \
+  "$SANDBOX/repo/skills-external/observability-and-instrumentation" \
   "$SANDBOX/repo/skills" "$SANDBOX/home/.claude"
 cp "$HELPER_SRC" "$SANDBOX/repo/lib/toggle-external.sh"
 # mark emil-design-eng ENABLED in the real (physical) repo tree
@@ -24,5 +25,11 @@ ln -s "$SANDBOX/repo/lib" "$SANDBOX/home/.claude/lib"
 out="$(bash "$SANDBOX/home/.claude/lib/toggle-external.sh" status emil-design-eng)"
 check T1-repo-resolves-through-symlink "$out" enabled
 
+# Same case arm, generalized to $tool for the agent-skills trio (this PR) —
+# left unlinked, so it must resolve through the symlink as "disabled", not
+# "missing" (which would mean REPO fell back to the wrong tree again).
+out="$(bash "$SANDBOX/home/.claude/lib/toggle-external.sh" status observability-and-instrumentation)"
+check T2-generalized-tool-resolves-through-symlink "$out" disabled
+
 rm -rf "$SANDBOX"
 printf 'PASS=%s FAIL=%s\n' "$pass" "$fail"; [ "$fail" -eq 0 ]