fix(onboard): route onboard under ns-project and drop from core profile

Integrate the brownfield /gsd:onboard skill into the skill subsystems so the
full CI suite passes:

- Route onboard under commands/gsd/ns-project.md (requires + routing row) so it
  nests as gsd-ns-project/skills/onboard on nested-layout runtimes instead of
  leaking as a 7th top-level skill dir (fixes install-nested-layout + issue-69).
- Remove onboard from PROFILES.core (src/install-profiles.cts) so the frozen
  main-loop core stays at 8 skills; onboard remains in standard/full.
- Add the TEXT_MODE plain-text fallback note to gsd-core/workflows/onboard.md
  for non-Claude runtimes (#2012).
- Allowlist onboard.md as a user-invocable skill (enh-2790 ratchet).
- Regenerate docs/INVENTORY-MANIFEST.json, golden-install-parity fixtures, and
  the workflow size baseline to match.

Co-authored-by: Codesmith <codesmith-bot@users.noreply.github.com>

src/install-profiles.cts

@@ -40,7 +40,6 @@ const {
 const PROFILES = Object.freeze({
   core: Object.freeze([
     'new-project',
-    'onboard',
     'discuss-phase',
     'plan-phase',
     'execute-phase',