[BUG] Hosted GitHub account sign-in returns a 404

Summary

The hosted OpenSRE account flow cannot complete GitHub sign-in: after selecting GitHub, GitHub renders its 404 page instead of authorizing and returning to OpenSRE.

Expected vs actual behavior

Expected: uv run opensre should complete the hosted account sign-in flow and return to the local CLI callback.

Actual: the CLI reaches the hosted sign-in flow, but GitHub returns a 404 for the authorization request handled through https://clerk.opensre.com/v1/oauth_callback. The local CLI remains blocked waiting for sign-in.

The OpenSRE account-start endpoint is reachable, so this appears to be in the hosted Clerk/GitHub social-connection configuration rather than in the local checkout. The GitHub OAuth client/redirect configuration should be verified by a maintainer with access to that configuration.

Steps to reproduce

1. From a fresh local checkout, run uv run opensre.
2. Choose the hosted account sign-in option.
3. In the OpenSRE sign-in page, select GitHub.
4. Complete any GitHub sign-in prompt.
5. Observe GitHub's 404 This is not the web page you are looking for page rather than a redirect back to OpenSRE.

Can you reproduce it consistently?

Yes.

How often does it occur?

Every time.

Operating system

Linux.

Logs and error output

No local Python exception is emitted; the browser flow stops at GitHub's 404 page.

Additional context

• The current hosted sign-in page advertises GitHub and Slack; GitHub is the failing option.
• This also blocks the interactive shell because hosted account sign-in is currently required (related: #6112).
• Screenshot follows in a comment. The OAuth state value is intentionally omitted from this report.