feat(docs): add "Copy as Markdown" button to docs pages

[ngnijland]

What

Adds a Copy as Markdown button to every docs content page. It sits above the page title, right-aligned, and copies a clean Markdown version of the page to the clipboard with brief "Copied!" feedback — so developers and LLMs can grab a page easily.

Placement/appearance is modelled on the starlight-copy-button plugin, but the Markdown is generated by reusing our existing llms.txt conversion rather than a new client-side HTML→Markdown converter (no new dependencies).

How

• src/lib/llms/page-markdown.ts (new) — extracts the content-level Markdown transforms (<Youtube>/<Tweet> → plain links, JSX-artifact stripping, ::: directives → blockquotes, etc.) into a shared module, plus docBodyToMarkdown(entry) that composes a page exactly like the llms generator (# title / > description / body).
• src/integrations/llms-txt-post-process.ts — refactored to reuse those helpers, so the button and the llms export produce identical output. Also fixes a latent gap: the <Youtube url="…"> variant was previously left as raw JSX in llms-full.txt; now both id and url variants convert.
• src/components/starlight/PageTitle.astro — renders the button, embeds the page Markdown inline (works identically in astro dev and production, no extra route), scoped theme-matched styles, an inline copy script (with a non-secure-context fallback), a visually-hidden aria-live region for screen readers, and a Plausible Copy as Markdown event on success.

Note on approach

Initially implemented as a static .md endpoint, but Starlight's /docs/* catch-all shadows it in astro dev (404), so the Markdown is embedded inline instead — largest page ~12.6 KB, most 2–6 KB, gzips to a few KB.

Verification

• Quality gate all green: lint, format:check, typecheck (0 errors), build, knip.
• Button present on content pages, absent on the docs index.
• Copied Markdown is clean; <Youtube> (both variants) renders as [title](youtube link), not raw markup.
• Inline JSON payload round-trips through JSON.parse (checked on the 12.6 KB code-heavy CLI page).

Before/after screenshot to be added.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a “Copy as Markdown” control to Starlight docs pages by generating per-page Markdown using the same transforms as the llms*.txt export, so users can quickly copy a clean Markdown representation of a page.

Changes:

• Introduces shared Markdown transform utilities and a docBodyToMarkdown(entry) composer for docs entries.
• Refactors llms-txt-post-process to reuse the shared transforms to keep outputs identical.
• Updates the custom PageTitle component to render the Copy button, embed page Markdown as JSON, and add client-side copy + feedback + analytics.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
docs/src/lib/llms/page-markdown.ts	New shared Markdown transform module + page-to-Markdown composer used by both the button and llms export.
docs/src/integrations/llms-txt-post-process.ts	Refactors llms post-processing to call the shared transforms.
docs/src/components/starlight/PageTitle.astro	Renders the “Copy as Markdown” button, embeds JSON payload, and adds inline copy script/styles.

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