fix(desktop): remove Buzz entity link previews

[tellaho]

Category: improvement
User Impact: Buzz-native project, repository, issue, and pull request links now appear once as compact inline chips, with their details available on hover.

Problem: Buzz-native entity links rendered both an inline chip and a standalone preview card, repeating the same metadata and adding visual noise to conversations. Solution: Exclude Buzz-native links from the shared standalone-preview extractor while leaving entity parsing intact for chip tooltips and preserving external web previews and attachment cards.

File changes

desktop/src/shared/lib/linkPreview.ts
Stops Buzz-native preview candidates after parsing, including same-relay git clone URLs that normalize to repository entities, while allowing external URLs through the existing snapshot path.

desktop/src/shared/lib/linkPreview.test.mjs
Covers project, repository, issue, pull request, markdown-labeled, same-relay clone, and mixed external-link extraction behavior.

desktop/src/shared/ui/markdown/useMessageLinkPreviews.test.mjs
Confirms sent messages no longer merge a standalone Buzz entity card while external sender snapshots still render.

Reproduction steps

1. Open a desktop channel containing a buzz://project, buzz://repo, buzz://issue, or buzz://pr link.
2. Confirm the link renders as an inline entity chip without a second standalone Buzz card below the message.
3. Hover the chip and confirm its entity metadata remains available.
4. Post an external HTTPS link and confirm its web preview still renders.
5. Paste a same-relay /git/<owner>/<repo> clone URL and confirm it uses the repository chip without a duplicate card.

Screenshots

Before	After
Inline chip plus redundant standalone Project card	Inline chip is now the sole presentation

After — rich metadata stays available on hover

Verification

At commit 3fa74cdd342ac1f6721b7d56a7f111af31e0e6e9:

• focused link-preview + Markdown unit suites — 119/119 passed
• targeted registered smoke E2E — 8/8 passed, including labeled same-relay clone metadata, ordinary-link presentation, and in-app navigation
• cd desktop && pnpm exec tsc --noEmit — passed
• git diff --check origin/main...HEAD — passed
• pre-push hooks — desktop check, TypeScript, and full desktop unit suite passed