Redirect to Bluesky-proper if opened in a browser

Just throwing out an idea here. Might not be what bsky.link wants to do at all.

[jort.link](https://jort.link/) has an interesting system where it serves a version of a link hosted by them only when requested by fediverse platforms, otherwise it returns a redirect to the actual link.

bsky.link could do something similar when people are using it to get embeds on Discord, Slack, etc: when a link is requested by [a link preview agent](https://tantek.com/2019/351/t1/link-preview-agent-not-microbrowser), return HTML with `` elements for preview generation, but if requested by a web browser redirect the user to Bluesky-proper.

Another prior art for a service like this is https://mstdn.link/ ([source](https://gist.github.com/thillsman/e0d755f114aef178d38e3d656385323d)). This one did no fancy user-agent detecting, instead putting a little bit of JS on the served page that handles a redirect.

No files or tests are named. Start by inspecting how bsky.link currently generates embeddable previews, then compare the proposed browser and link-preview-agent behavior with jort.link and mstdn.link. Done would require agreed behavior for browser redirects and preview HTML, plus a way to verify both request types.

Written by the indexing model from the issue text.