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

bin/check-index

@@ -0,0 +1,42 @@
+#!/bin/sh
+# check-index — asserts SKILLS_INDEX.md lists exactly the skills that exist.
+#
+# The index is hand-maintained and drifts silently in both directions: a skill
+# ships with no row (`dream` and `experiment-spec` did — their rows lived on
+# branches squash-merged after a directory reorg, so the SKILL.md landed and
+# the row did not, #15), or a row outlives the skill it names. Neither errors
+# anywhere; the library just misreports itself to every harness that reads it.
+#
+# Usage: check-index [checkout-root]   (defaults to this script's repo root)
+
+set -u
+
+ROOT=${1:-$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)}
+INDEX="$ROOT/SKILLS_INDEX.md"
+fail=0
+
+if [ ! -f "$INDEX" ]; then
+  echo "ERROR: missing SKILLS_INDEX.md" >&2
+  exit 1
+fi
+
+# A row is a table line whose first cell is the skill name in backticks.
+listed=$(sed -n 's/^|[[:space:]]*`\([^`]*\)`.*/\1/p' "$INDEX" | sort -u)
+
+for dir in "$ROOT"/skills/*/; do
+  name=$(basename "$dir")
+  [ -f "$dir/SKILL.md" ] || continue
+  printf '%s\n' "$listed" | grep -qx "$name" || {
+    echo "ERROR: skills/$name has no row in SKILLS_INDEX.md" >&2
+    fail=1
+  }
+done
+
+for name in $listed; do
+  [ -f "$ROOT/skills/$name/SKILL.md" ] || {
+    echo "ERROR: SKILLS_INDEX.md lists '$name', which has no skills/$name/SKILL.md" >&2
+    fail=1
+  }
+done
+
+exit $fail