refactor(#887): consolidate duplicated CHILD_ROUTER test maps into shared helper (#889)

Three test files copy-pasted the concrete-skill->namespace-router CHILD_ROUTER
map verbatim, and two more duplicated an identical local parseRouterRequires
regex. Introduce tests/helpers/nested-layout.cjs that derives the child->router
map from the authoritative commands/gsd/ns-*.md requires: lists once, reusing
the production parseRequires (now exported from install-profiles) plus a
nestedSkillPath(skillsRoot, prefix, stem) helper. Refactor all five test files
to import from it.

Test-only + a single internal export addition; no production behavior change.

Closes #887

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/install-profiles.cts

@@ -720,6 +720,7 @@ export = {
   readActiveProfile,
   writeActiveProfile,
   // Shared internals
+  parseRequires,
   cleanupStagedSkills,
   // Back-compat / deprecated
   MINIMAL_SKILL_ALLOWLIST,