Skill authoring: skill-creator meta-skill + audience-structured skills

[sachiniSam]

Summary

Ship the skill-creator meta-skill (authoring conventions for skills) and restructure the multi-audience platform skills into the shared-core + audience-references shape it prescribes.

Spec: docs/superpowers/specs/2026-07-21-skills-experience-design.md §2 (audience-structured skills, no phase field).

Scope

• skill-creator skill in the platform library: trigger-rich descriptions naming all audiences, shared-core body + per-audience references/*.md layout, frontmatter contract (identity + human labels only; no phase; machine bookkeeping lives in skills-manifest.json), size budgets. Flows like any platform skill (seeded, reconciled, override-able).
• Restructure multi-audience skills — at minimum excalidraw-wireframes (authoring.md for the design agent writing wireframes.dsl; implementing.md for the coding agent translating the DSL); assess openapi-conventions and cell-architecture-dsl for the same split.
• Descriptions across the library re-checked: every trigger/audience named (a build-only skill's description says "Apply to every X component").

Acceptance

• skill-creator present in the embedded library and seeded to orgs.
• excalidraw-wireframes has shared-core body + both audience references; description names both uses.

Part of #292.