GitHub @mentions corrupted to [From GitHub: user](url) on sync #9071

Affected: Plane Cloud, official GitHub integration. See also: meta-issue on destructive overwrite (issue 01).

Symptom

GitHub markdown @username written in an issue or comment body, after a GH→Plane import + Plane→GH round-trip, comes back as [From GitHub: username](https://github.com/username).

The From GitHub: <handle> text is injected at GH→Plane import time by the closed makeplane/silo-commercial image. The Plane→GH markdown serialisation then faithfully emits it.

Impact

• Loses GitHub-native mention semantics: no notification to the mentioned user, no avatar in GH rendering, no auto-link styling.
• Visually ugly: @user becomes From GitHub: user everywhere.
• Words that look like mentions but aren't valid users still get linked to broken URLs. e.g. the literal text @mentions becomes [From GitHub: mentions](https://github.com/mentions) — a 404.

Ask

Preserve @user mentions as bare @user text across the round-trip. Do not invent [From GitHub: user](url) link forms.

If conversion to a link is unavoidable, validate the handle against GitHub's user API and skip the rewrite for non-existent users rather than producing a broken link.

Reproduction

1. Connect a GitHub repo to a Plane workspace via the official GitHub integration. Set sync to bidirectional.
2. Create a GitHub issue with body cc @someuser. Apply the Plane label.
3. Within seconds the Plane work item appears with description_html already containing <a href="https://github.com/someuser">From GitHub: someuser</a>.
4. To exercise the destructive Plane→GH push, follow the trigger recipe in meta-issue 01.