ci(publication): enforce placeholder policy remotely

[stacknil]

Summary

• run the canonical placeholder validator in the main Markdown Validation workflow
• trigger the workflow when the validator implementation changes
• keep the existing manual artifact audit as a separate non-blocking maintenance surface

Design decision

Use the existing full-corpus validator as the remote gate. The current 157-note corpus is clean, so no debt exception or duplicate changed-file implementation is required.

Main risk

This check enforces placeholder vocabulary only. It does not replace human publication review, secret scanning, or authorization review.

Compatibility impact

No note schema or public content changes. Contributors may now receive a failing PR check for noncanonical placeholders that local pre-commit already rejected.

Rollback path

Revert this single workflow commit to restore the previous CI behavior.

Verification

• python scripts/render_tags_doc.py --check
• python scripts/render_readme_snapshot.py --check
• python scripts/check_pattern_library.py
• python scripts/check_placeholders.py
• python scripts/check_markdown.py
• python -m pre_commit run --files .github/workflows/markdown-lint.yml
• git diff --check

[stacknil]

Post-CI diff review complete.

• Design decision: run the existing full-corpus placeholder validator in the main Markdown Validation job and trigger that workflow when the validator changes.
• Main risk: this gate validates placeholder vocabulary only; human publication review, authorization checks, and secret scanning remain separate responsibilities.
• Compatibility impact: no content or schema changes. Remote CI now enforces a policy that local pre-commit already enforced.
• Rollback path: revert commit 309c6ce to remove the workflow step and trigger entries.

Evidence: the reviewed local and remote heads match; the diff is one workflow file with five added lines; git diff --check and privacy scanning pass; both workflow runs are green; and the GitHub job log confirms the new placeholder-validation step completed successfully.