docs: one page for a coding assistant, and a copy-page control on every page

[AlexeyShalaev]

Three things, one docs change.

A "Copy page" control above every page. A split button that copies the page's
Markdown, or opens it in ChatGPT, Claude or Perplexity. It works because
scripts/emit_markdown.py runs after the build and writes every docs/<path>.md next
to the HTML it built, so each page has a plain-text twin one URL away — /agents/ and
/agents.md. The Docs workflow gained that one step.

docs/agents.md — the whole library on one page, for a coding assistant: the facts
table, the arithmetic timeout_for actually does, both classes and both contrib modules,
fifteen rules, the mistakes a model makes with this API, and a map of which page to fetch
for the rest. Two sections exist because they are what a model gets wrong here and the
library gives no error when it does:

• Crossing a process boundary — time.monotonic() is process-local, so the budget
  object never travels; the number from timeout_for does, and the callee builds its own
  budget from it. There is no header, no context variable and nothing ambient: in-process
  propagation is a function argument.
• When the budget runs out — nothing fires. No timer, no cancellation. remaining()
  goes negative, the next timeout_for raises, an in-flight call keeps running on the
  timeout it was already given, and a downstream timeout still raises the client's own
  error rather than ours.

The rule that keeps it true. CONTRIBUTING.md and the pull request template now say
the page is part of the public API and changes in the same pull request the API does. A
stale one is worse than none — it teaches a model an API that no longer exists.

docs/reference/index.md takes copy_page: false: its Markdown is two lines of
instruction to mkdocstrings, not the API, so it carries neither the control nor a twin.

How to see it. The Docs workflow deploys on merge; after that,
bedrock-python.github.io/deadline-budget/agents/ and the button at the top of every
page. Locally: make docs-build && uv run --no-dev --group docs python scripts/emit_markdown.py,
then serve site/ — zensical serve rebuilds into the same directory without the twins
and answers 404 to the control.

docs/assets/stylesheets/copy-page.css, docs/assets/javascripts/copy-page.js,
overrides/main.html and scripts/emit_markdown.py are byte-identical across the
organisation, so they can be updated in one sweep.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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