Improve validate-layout PR comment with pass/fail details

## Summary
`validate-layout.yml` posts a generic PR comment. Contributors need field-level validation output.

## Tasks
- [ ] Capture `slideforge validate` stdout/stderr per layout
- [ ] Post a markdown table: layout ID, status, errors
- [ ] Only comment on failure, or always comment with summary

## Files
- `.github/workflows/validate-layout.yml`
- `packages/sdk/src/validate.ts`

Start with .github/workflows/validate-layout.yml to trace how slideforge validate is run and how the current pull-request comment is posted, then read packages/sdk/src/validate.ts for the available validation output. Run validation against the layouts covered by the workflow. Done means the PR comment reports each layout ID, pass/fail status, and errors, with the failure-only or summary behavior made consistent.

Escrita pelo modelo de indexação a partir do texto da issue.