Reject self-pairing the local Orca server as a remote

The local Orca server can be added as its own remote environment, creating an invalid self-pair. Reimplement the explicit self-pairing guard with runtime and UI coverage. Source PR https://github.com/stablyai/orca/pull/9137; credit @s546126. No contributor action requested.

Start by reviewing source PR #9137, which is identified as the implementation reference; the issue names no files or entry points. Confirm that the explicit self-pairing guard is covered by both runtime and UI tests, with self-pairing rejected and no invalid remote created.

Written by the indexing model from the issue text.