refactor(ci): use RPM_REPO_HOST env var, add SSH connectivity test

- Set RPM_REPO_HOST=oolon.kosherinata.internal as a plain env var
  instead of treating the hostname as a secret via RSYNC_TARGET
- Add explicit SSH connectivity test step using StrictHostKeyChecking=accept-new
- Remove ssh-keyscan in favour of accept-new which provides meaningful errors
- Remove RSYNC_TARGET secret dependency

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

readme.md

@@ -117,5 +117,4 @@ The build-release workflow requires the following secrets:
 | `DISPATCH_TOKEN` | Gitea API token for triggering builds         |
 | `RPM_SIGNING_KEY`| ASCII-armored GPG signing subkey              |
 | `RPM_SIGNING_KEY_ID` | GPG key UID (`rpm@lair.cafe`)            |
-| `RSYNC_TARGET`   | SSH target for rsync (e.g. `gitea_ci@oolon`)  |
 | `RSYNC_SSH_KEY`  | SSH private key for the `gitea_ci` user       |