[joahg]

Linear: AGNTOPS-773

Why

Core Buzz should unfurl links, and plugins should be able to override the default per site (Joah, plugin bounty thread). Plugins have no slot below a message today. The marketplace link-previews plugin renders its cards inside the link anchor, then moves the DOM nodes under the bubble and watches them with MutationObservers.

What

• ctx.conversation.registerLinkPreview({ id, title, matches(url), order?, component }). Components receive { url, messageId, open }. open() routes through the host's link handler, as clicking the link does, and returns false once the contribution or message retires.
• Override model: for each link the lowest order among matches draws its card, the same resolution registerLink and panels use. A default web card sits at the catch-all band (100), so a plugin claims its own site at 0. A component that renders nothing leaves that link without a card.
• Which links get cards (previewLinks): credential-free HTTPS links in the message's Markdown prose, once each in the order written. Bare and labeled links count. <url> doesn't, since it asks for no preview, matching Let authors dismiss link previews with a one-link edit #2677's convention. Neither do links in code, images, or links naming one of the message's attachments. Diffs, agent envelopes and messages drawn by a message renderer get none.
• MessageRow mounts the stack between the bubble and its attachments. After three cards, the rest fold behind "Show N more previews". Each card has its own ContributionBoundary, so a failing renderer drops only its card.
• Docs: plugin-architecture.md and plugin-authoring-guide.md.

No core card ships in this PR; the bundled buzz.link-previews plugin follows it in the stack.

Review notes

• contracts.ts and service.tsx are FOUNDATION. This adds one contribution kind beside registerLink, with the same validation and lifetime. Joah asked for this extension point explicitly.
• editedInBody in MessageRow is now derived from markdownBody, which the previews also use. Behavior is unchanged.

Testing

• vitest: preview-links.test.ts (new), LinkPreviews.test.tsx (new: order override, fold and unfold, host open, a throwing card isolated), service.test.tsx (new: registration validation and withdrawal), and the existing MessageRow tests: 223 passed.
• tsc --noEmit is clean.
• A browser spec that drives real message rows through this slot is in the bundled-plugin PR, since it needs a card.

Part of a stack:

1. feat(plugins): let link previews read a public page's preview fields #3217: page reads.
2. This PR: the slot.
3. Bundled buzz.link-previews, on by default.

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