👷 ci: adopt shared message validation

[zrr1999]

Summary

• Replace the duplicated PR body parser with the upstream ZenDev action and shared Markdown scanner. A ~~~ fenced example can contain H2 text without adding a PR section.
• Migrate the commit hook to zendev-lab/zendev and the supported zendev-message-check entry. Pin the hook and both PR actions to the same upstream revision.
• Include pnpm-workspace.yaml in the documented Cloudflare build watch paths for all three Workers.

Validation

• prek run --all-files and the installed commit hook pass, including actionlint, Markdown/TOML checks, links, and workspace Astro checks.
• A body with H2 text inside a ~~~ fence passes; a body missing required Validation and Notes sections fails.
• The underlying website build and Chromium search/navigation checks pass in the preceding PR.

Notes

Depends on #14. Shared validation is pinned to 7d8bcbf6134f62b7c7eaadb763f9a0d434717c44; v0.4.0 still fails the fenced-heading case. The English PR template remains the body contract.

Cross-repository migration: zrr.dev #85 uses the same pinned upstream validation revision.