feat: support disableAutolinkProtocols to opt out of GFM autolinks per protocol

[sleitor]

Closes #607.

What

Adds an optional disableAutolinkProtocols prop to <Streamdown> that disables GFM autolink-literal linking for specific URL protocols (e.g. mailto), as requested in #607.

<Streamdown disableAutolinkProtocols={["mailto"]}>
  {"Contact us at hello@example.com"}
</Streamdown>

Before:

<p>Contact us at <a href="mailto:hello@example.com">hello@example.com</a></p>

After (with disableAutolinkProtocols={["mailto"]}):

<p>Contact us at hello@example.com</p>

• Protocol names are case-insensitive and accept an optional trailing colon ("mailto" and "mailto:" are equivalent).
• Only literal autolinks created by remark-gfm's autolink-literal extension are affected (bare emails, bare http(s)/www URLs). Explicit markdown links ([text](mailto:...)) are left as links, since they're an intentional link, not an autolink.
• Default behavior is completely unchanged when the prop is omitted — no extra plugin is added to the remark pipeline in that case.

How

mdast-util-gfm-autolink-literal doesn't tag the link nodes it creates, so a small remark plugin (packages/streamdown/lib/remark/disable-autolink-protocols.ts) identifies them structurally: a link with exactly one text child whose value reconstructs the node's url (accounting for the mailto: prefix GFM adds to bare emails). Matching nodes are unwrapped back to their text children. The plugin runs immediately after remark-gfm in the pipeline (before the CJK/math plugin slots), matching the existing "before/after gfm" plugin ordering convention.

It's exposed via the standard unified [plugin, options] tuple form rather than a plugin-factory closure — Streamdown's internal Markdown processor cache keys processors by plugin name + JSON.stringify(options), and a factory returning a fresh anonymous closure per call would always serialize to the same cache key, causing different disableAutolinkProtocols configurations to silently reuse a stale cached processor (caught this via a failing test while developing).

Testing

Added packages/streamdown/__tests__/disable-autolink-protocols.test.tsx covering:

• bare email not linked when mailto is disabled
• default behavior (prop omitted) unchanged
• explicit [text](mailto:...) links stay linked
• http/https autolinks still linked when only mailto is disabled
• case-insensitivity and the "mailto:" form
• disabling https leaves mailto autolinks untouched
• end-to-end wiring through the <Streamdown disableAutolinkProtocols> prop itself

pnpm -F streamdown exec vitest run passes except for 5 pre-existing failures on main unrelated to this change (image-placeholder / incomplete-image-sanitize tests, verified failing identically on a clean upstream/main checkout before my changes).

Also updated apps/website/content/docs/configuration.mdx with the new prop, and added a changeset.

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