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

Taskfile.yml

@@ -94,7 +94,12 @@ tasks:
           count=$((count + 1))
         done
 
-        # 3. Plugin manifest version must match the latest tag, or installers
+        # 3. SKILLS_INDEX.md lists exactly the skills that exist. Delegated
+        #    to bin/check-index so the rule is covered by
+        #    tests/check_index_test.sh rather than living untested here.
+        sh '{{.TASKFILE_DIR}}/bin/check-index' '{{.TASKFILE_DIR}}' || fail=1
+
+        # 4. Plugin manifest version must match the latest tag, or installers
         #    never see the update (Claude Code gates updates on this field).
         if [ -f '{{.PLUGIN_MANIFEST}}' ]; then
           manifest_v=$(sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' '{{.PLUGIN_MANIFEST}}' | head -1)