feat: new libraries ship the agents page and the copy-page control

[AlexeyShalaev]

Every library in the organisation is getting two things this week: docs/agents.md, the one page you hand a coding assistant instead of the whole site, and a Copy page control that hands any page to a chat window as Markdown. A library generated from this template should not have to be retrofitted the day after it is created, so it now starts with both.

What a generated project gets

• docs/assets/javascripts/copy-page.js, docs/assets/stylesheets/copy-page.css, overrides/main.html, scripts/emit_markdown.py — the four shared files, byte-identical to the ones in every other repository, wired into zensical.toml and the Docs workflow. The script writes each page a second time as raw Markdown next to its HTML, which is what the control fetches.
• docs/agents.md — a skeleton with the sections that carry weight and TODO where the author has to write. It is in the nav, so a build fails while it is missing rather than shipping an empty promise.
• docs/reference/index.md declines both with copy_page: false: its Markdown is instructions to a docstring renderer, not the API.
• CONTRIBUTING.md says the page 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.

What the author is told

NEW_LIBRARY_CHECKLIST.md gains a step right after the local setup, and .claude/LIBRARY_CREATION.md a section of its own, both saying the same thing: fill the page in from the API you actually wrote, verify every name it tells a caller to import, delete the guidance comment, and do not commit the TODO markers. The clean-repository checklist and the common-mistakes list now cover it too.

Verified by generating one. copier copy into a scratch directory, then zensical build --clean reports no issues, emit_markdown.py writes 6 pages with 1 declined, and the built HTML carries data-copy-site="demo-lib" — the control takes the project's name from config.site_name, which is why the script is the same file everywhere.