feat(validate): fail build on unsubstituted {{placeholders}} in rendered Markdown

[mvanhorn]

Summary

• Extend lib/validate.ts to walk dist/**/*.md and fail the build if any unsubstituted {{[A-Za-z0-9_]+}} placeholders remain in rendered Markdown.
• Mirrors the existing jsonFiles() helper style; no new dependencies, no changes to lib/build-helpers.ts or any lib/strategies/*.ts.
• Catches the failure mode where a new placeholder is added to src/content/*.md but not registered in RenderOptions + render(), which otherwise ships literal {{NEW_PLACEHOLDER}} text in agent-visible skill bodies.

Validation

npm run check passes on current main (current rendered output is clean).

Demo of the check catching a regression ({{FAKE_PLACEHOLDER}} appended to src/content/skill.md):

$ npm run build && npm run validate
> tsx lib/validate.ts

Error: Rendered Markdown contains unsubstituted placeholders. Add the missing placeholders to render() in lib/build-helpers.ts (or remove them from src/content/*.md):
  dist/claude-code/replayio/skills/replayio/SKILL.md: {{FAKE_PLACEHOLDER}}
  dist/codex/replayio/skills/replayio/SKILL.md: {{FAKE_PLACEHOLDER}}
  dist/cursor/replayio/skills/replayio/SKILL.md: {{FAKE_PLACEHOLDER}}

AI was used for assistance.