Long-term: migrate from peaceiris/actions-gh-pages to actions/deploy-pages

## Summary
`peaceiris/actions-gh-pages` is currently used to publish to GitHub Pages. It works fine and is actively maintained, but:
- It is a single-maintainer action (5.3k stars, well-known but not GitHub-verified).
- Last release was 2024-04-08 — over 2 years between releases despite recent commit activity.
- It receives `GH_TOKEN` and writes to the `gh-pages` branch.

## Suggested direction
GitHub now provides first-party actions for this purpose:
- `actions/upload-pages-artifact`
- `actions/deploy-pages`

These use the modern Pages deployment API rather than pushing to a `gh-pages` branch, which is the path GitHub itself recommends.

## Scope
Used in `specification-github-pages.yaml` (and possibly elsewhere — check with `grep -l peaceiris .github/workflows/`).

## Why "long-term"
Not urgent — current usage is fine. This is a strategic migration to reduce reliance on individual-maintainer actions and align with GitHub's recommended path. Bundle with the next significant change to the spec-publishing flow.

## Context
Surfaced during a security audit of third-party actions. Tagged Medium risk; logged as a slow-burn improvement.