docs: the agents page is part of the public API, and one shared copy-page script

[AlexeyShalaev]

This is the tail of a rollout: docs/agents.md and the Copy page control started here, and are now going into every library in the organisation. Two things follow from that.

The rule was never written down. The page is the one people hand to an assistant instead of the site, so a stale one is worse than none — it teaches a model an API that no longer exists. CONTRIBUTING.md now says it is part of the public API and changes in the same pull request the API does, and the pull request checklist names it on the documentation line. The new section also records how the Markdown twins get written and why the API reference declines them, which was folklore until now.

The script had this library's name baked in. copy-page.js built its prompt around the literal string pg-partsmith, which meant every other repository would carry a near-copy differing in one word. The name now comes from config.site_name through a data-copy-site attribute, so the four shared files — the script, the stylesheet, overrides/main.html and scripts/emit_markdown.py — are byte-identical everywhere and can be updated in one sweep.

No behaviour change: the control renders and reads the same, and the prompt it builds is the same sentence it was.

Verified with a full build: zensical build --clean reports no issues, emit_markdown.py writes 45 pages with 1 declined, and the built HTML carries data-copy-site="pg-partsmith". uv.lock untouched.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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