fix(auth): point SSH OAuth users at the tunnel they actually need (#26592)

Two loopback-redirect OAuth flows (xAI Grok, Spotify) silently fail when
Hermes runs on a remote host: the auth server redirects to
127.0.0.1:<port> on the user's laptop, not on the remote box. The
--no-browser flag only suppresses webbrowser.open() — it doesn't change
the bind address. Symptom xAI surfaces is 'Could not establish
connection. We couldn't reach your app.', followed by a 'xAI
authorization timed out waiting for the local callback' on the CLI side.

Changes
- hermes_cli/auth.py: new _print_loopback_ssh_hint() helper, called from
  _xai_oauth_loopback_login() and _spotify_login() right after they
  print the redirect URI. Silent off SSH; on SSH prints the exact
  'ssh -N -L <port>:127.0.0.1:<port>' command using the actually-bound
  port (not the hardcoded constant — the listener auto-bumps when the
  preferred port is busy), a provider-specific docs URL, and a link to
  the new shared guide.
- website/docs/guides/oauth-over-ssh.md (new): single source of truth
  for the tunnel pattern — TL;DR command, jump-box / ProxyJump variant,
  mosh+tmux+ControlMaster gotchas, troubleshooting.
- website/docs/guides/xai-grok-oauth.md: fix the two sections that
  claimed --no-browser alone was enough; link to the shared guide.
- website/docs/user-guide/features/spotify.md: expand the existing
  one-liner; link to the shared guide.
- website/sidebars.ts: register the new page.
- tests/hermes_cli/test_auth_loopback_ssh_hint.py: 7 unit tests
  covering SSH-vs-not, loopback-vs-not, malformed URIs, port echo,
  with and without provider docs URL.

website/sidebars.ts

@@ -192,6 +192,7 @@ const sidebars: SidebarsConfig = {
         'guides/aws-bedrock',
         'guides/azure-foundry',
         'guides/xai-grok-oauth',
+        'guides/oauth-over-ssh',
         'guides/microsoft-graph-app-registration',
         'guides/operate-teams-meeting-pipeline',
       ],