docs(index): add dream + experiment-spec rows, and gate the index

Both skills shipped without an index row. The rows existed — on the feature
branches deleted in #22 — and were dropped by the squash-merge that followed
a directory reorg: the SKILL.md landed under skills/, the row did not. Nothing
errored, the index just under-reported the library to every harness reading it.

Adding two rows by hand fixes today and not tomorrow, so the assertion goes in
`task check` via bin/check-index, following the same pattern as
bin/should-skip-release: a real script with a real test, not untested logic
buried in a Taskfile. It checks both directions — a skill with no row, and a
row naming a skill that no longer exists.

Verified by mutation: deleting the `dream` row turns `task check` red and the
report names it.

Closes #15
Bump-Type: patch

tests/check_index_test.sh

@@ -0,0 +1,70 @@
+#!/bin/sh
+# Tests for bin/check-index.
+#
+# The regression these exist for: SKILLS_INDEX.md is maintained by hand, and
+# two skills (`dream`, `experiment-spec`) shipped without a row — the index
+# rows lived on feature branches that were squash-merged after a directory
+# reorg, so the SKILL.md landed and the row did not (#15, #22). Nothing
+# errored; the index just quietly under-reported the library.
+
+set -u
+
+ROOT=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+SUT="$ROOT/bin/check-index"
+
+pass=0
+fail=0
+WORK=${TMPDIR:-/tmp}/check_index_test.$$
+
+if [ ! -f "$SUT" ]; then
+  printf 'FAIL: %s does not exist\n' "$SUT"
+  exit 1
+fi
+
+cleanup() { rm -rf "$WORK"; }
+trap cleanup EXIT INT TERM
+
+# new_fixture <name> — a checkout with skills/alpha and skills/beta.
+new_fixture() {
+  _f="$WORK/$1"
+  mkdir -p "$_f/skills/alpha" "$_f/skills/beta"
+  printf -- '---\nname: alpha\ndescription: a\n---\n' > "$_f/skills/alpha/SKILL.md"
+  printf -- '---\nname: beta\ndescription: b\n---\n' > "$_f/skills/beta/SKILL.md"
+  printf '%s' "$_f"
+}
+
+index() {
+  _dir=$1; shift
+  printf '# Engineering Skills Index\n\n| Skill | Description | Use when |\n|---|---|---|\n' > "$_dir/SKILLS_INDEX.md"
+  for _n in "$@"; do
+    printf '| `%s` | desc | when |\n' "$_n" >> "$_dir/SKILLS_INDEX.md"
+  done
+}
+
+# expect <0|1> <label> <fixture> [grep-for]
+expect() {
+  _want=$1; _label=$2; _fx=$3; _needle=${4:-}
+  _out=$(sh "$SUT" "$_fx" 2>&1); _rc=$?
+  if [ "$_rc" -ne "$_want" ]; then
+    fail=$((fail + 1)); printf '  FAIL %s (want rc=%s got %s)\n%s\n' "$_label" "$_want" "$_rc" "$_out"; return
+  fi
+  if [ -n "$_needle" ] && ! printf '%s' "$_out" | grep -q "$_needle"; then
+    fail=$((fail + 1)); printf '  FAIL %s (rc ok, but report never names %s)\n%s\n' "$_label" "$_needle" "$_out"; return
+  fi
+  pass=$((pass + 1)); printf '  ok   %s\n' "$_label"
+}
+
+fx=$(new_fixture complete); index "$fx" alpha beta
+expect 0 'every skill has a row' "$fx"
+
+fx=$(new_fixture missing); index "$fx" alpha
+expect 1 'a skill with no row is reported' "$fx" beta
+
+fx=$(new_fixture stale); index "$fx" alpha beta gamma
+expect 1 'a row for a skill that does not exist is reported' "$fx" gamma
+
+fx=$(new_fixture noindex)
+expect 1 'a missing index is reported' "$fx" SKILLS_INDEX.md
+
+printf '\ncheck_index: %d passed, %d failed\n' "$pass" "$fail"
+[ "$fail" -eq 0 ]