refactor(skills): progressive disclosure for release-readiness-gate (references + templates)

[rekhako]

Summary

Follow-up from the #2422 review (F4, Katrien De Graeve (@katriendg)). Apply progressive disclosure to the release-readiness-gate skill so SKILL.md stays compact, per the prompt-builder guidance.

Proposed changes

• Move the Default Pillars table to references/pillars.md, loaded on demand.
• Move the scorecard template (Output Format) to templates/scorecard.md.
• Reference both from SKILL.md instead of inlining them.

Context

• Deferred from feat(skills): add release-readiness-gate skill and release-readiness collection #2422 to keep that PR focused on collection placement (F1) and section structure (F3).
• Severity: Low.

Acceptance criteria

• SKILL.md references references/pillars.md and templates/scorecard.md instead of inlining that material.
• npm run validate:skills and npm run lint:frontmatter pass.
• Plugins regenerated via npm run plugin:generate.