docs(xai-oauth): add xai-oauth to provider enumeration pages (#26542)

Follow-up to #26534 (xai-oauth provider). The new guide and integrations
page were shipped with the salvage, but four reference/enumeration pages
still listed every other OAuth provider without xai-oauth:

- reference/cli-commands.md     — `--provider` choices list
- reference/environment-variables.md — HERMES_INFERENCE_PROVIDER values
- user-guide/configuration.md   — auxiliary-task provider list, OAuth
                                  tip block (mirrored from MiniMax OAuth),
                                  and provider table row
- user-guide/features/fallback-providers.md — provider table

website/docs/user-guide/features/fallback-providers.md

@@ -66,6 +66,7 @@ Both `provider` and `model` are **required**. If either is missing, the fallback
 | Google Gemini (OAuth) | `google-gemini-cli` | `hermes model` (Google OAuth; optional: `HERMES_GEMINI_PROJECT_ID`) |
 | Google AI Studio | `gemini` | `GOOGLE_API_KEY` (alias: `GEMINI_API_KEY`) |
 | xAI (Grok) | `xai` (alias `grok`) | `XAI_API_KEY` (optional: `XAI_BASE_URL`) |
+| xAI Grok OAuth (SuperGrok) | `xai-oauth` (alias `grok-oauth`) | `hermes model` → xAI Grok OAuth (browser login; SuperGrok subscription) |
 | AWS Bedrock | `bedrock` | Standard boto3 auth (`AWS_REGION` + `AWS_PROFILE` or `AWS_ACCESS_KEY_ID`) |
 | Qwen Portal (OAuth) | `qwen-oauth` | `hermes model` (Qwen Portal OAuth; optional: `HERMES_QWEN_BASE_URL`) |
 | MiniMax (OAuth) | `minimax-oauth` | `hermes model` (MiniMax portal OAuth) |