Design save: validate skillsApplied against the org catalog

[sachiniSam]

Summary

Design save validates every skillsApplied[] entry against the org's resolved skill catalog — an unknown name returns a corrective error to the design agent instead of today's silent warn-and-skip at build time.

Spec: docs/superpowers/specs/2026-07-21-skills-experience-design.md §4.

Scope

• Validation in the design-save path: every skillsApplied name must resolve in the org catalog; unknown → corrective error (same pattern as SCHEMA_VIOLATION), listing the available names.
• CRT annotation auto-attach (attach_skills.go equivalent) stays tolerant (a PE typo must not brick saves) but reports unresolvable names in the save response.
• Runtime dangling-pin behavior (deletion after save) is covered by the runner-consumption issue; this issue is design-save time only.

Acceptance

• Saving a design with a typo'd skill name returns a corrective error naming the unknown skill and the catalog.
• A CRT-annotated unresolvable skill still saves, with the problem reported in the response.

Part of #292.