Wiring-fix: single source of truth for review leaf list + CI enforcement

De-enumerate the AL review leaf list so it lives in exactly one place:
the al-code-review super-skill's frontmatter sub-skills list.

- al-code-review.md: drop the enumerated parenthetical from description
  and remove the redundant bullet re-list in the Source section.
- README.md: remove the hardcoded count word and the enumerated domain
  list from the leaf-skill sentence.
- validate_frontmatter.py: add cross-file rule R26 asserting a
  super-skill's declared sub-skills exactly match the sibling
  al-*-review.md leaf files on disk (missing, stale, and unregistered
  leaves all fail CI).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/scripts/validate_frontmatter.py

@@ -504,9 +504,48 @@ class SkillRecord:
     skill_id: str | None
 
 
+def validate_sub_skills_registry(path: Path, fm: dict[str, Any], root: Path, report: Report) -> None:
+    """R26: a super-skill's declared `sub-skills` must exactly match the
+    `al-*-review.md` leaf files present in the same directory (set equality,
+    ordering-agnostic). This keeps the registered leaf list the single source
+    of truth and fails CI on a forgotten, stale, or missing registration.
+
+    Only applies to action-skill files declaring a non-empty list-of-str
+    `sub-skills`. Files whose `sub-skills` is malformed are handled by R20.
+    """
+    ss = fm.get("sub-skills")
+    if not is_non_empty_list_of_str(ss):
+        return
+
+    declared = {s.lstrip("./") for s in ss}
+
+    # Sibling leaves on disk, excluding the super-skill file itself.
+    leaves = {
+        p.relative_to(root).as_posix()
+        for p in path.parent.glob("al-*-review.md")
+        if p.resolve() != path.resolve()
+    }
+
+    # Declared entries that are not real sibling leaves on disk (missing/stale).
+    for entry in sorted(declared - leaves):
+        entry_path = root / entry
+        if not entry_path.exists():
+            report.error(path, "R26", f"declared sub-skill does not exist on disk: {entry}", 1)
+        else:
+            report.error(
+                path, "R26",
+                f"sub-skills entry is not a sibling 'al-*-review.md' leaf: {entry}", 1,
+            )
+
+    # Sibling leaves on disk that were never registered ('forgot to wire it up').
+    for leaf in sorted(leaves - declared):
+        report.error(path, "R26", f"leaf not registered in sub-skills: {leaf}", 1)
+
+
 def run(root: Path) -> Report:
     report = Report()
     skill_records: list[SkillRecord] = []
+    action_skill_fms: list[tuple[Path, dict[str, Any]]] = []
 
     # Walk declared top-level folders only; avoid wandering into .git, etc.
     walk_roots = [root / "skills"] + [root / layer for layer in LAYERS]
@@ -533,6 +572,8 @@ def run(root: Path) -> Report:
             validate_knowledge(path, parsed, report)
         elif kind == "action-skill":
             validate_action_skill(path, parsed, report)
+            if parsed.frontmatter:
+                action_skill_fms.append((path, parsed.frontmatter))
             if parsed.frontmatter and isinstance(parsed.frontmatter.get("id"), str):
                 skill_records.append(SkillRecord(path, "action-skill", parsed.frontmatter["id"]))
         elif kind == "meta":
@@ -566,6 +607,10 @@ def run(root: Path) -> Report:
                     others = [q.relative_to(root).as_posix() for q in paths if q != p]
                     report.error(p, "R24", f"skill id '{sid}' ({kind}) is not unique; also defined in: {others}")
 
+    # Fourth pass: R26 sub-skills registry matches leaf files on disk
+    for path, fm in action_skill_fms:
+        validate_sub_skills_registry(path, fm, root, report)
+
     return report