feat(ai): add xAI device OAuth and route grok-4.5 through Responses (#6651)

* feat(ai): add xAI device OAuth and route grok-4.5 through Responses

Add xAI device-code OAuth alongside XAI_API_KEY. Route only grok-4.5
through Responses with low/medium/high reasoning; other xAI models stay
on Completions.

* fix(ai): tolerate xAI device-code interval 0 and correct token poll error label

---------

Co-authored-by: Jaaneek <Jaaneek@users.noreply.github.com>

packages/coding-agent/docs/providers.md

@@ -18,6 +18,7 @@ Use `/login` in interactive mode, then select a provider:
 - ChatGPT Plus/Pro (Codex)
 - Claude Pro/Max
 - GitHub Copilot
+- xAI (Grok/X subscription)
 - Radius
 
 Use `/logout` to clear credentials. Tokens are stored in `~/.pi/agent/auth.json` and auto-refresh when expired.
@@ -36,6 +37,11 @@ Anthropic subscription auth is active for Claude Pro/Max accounts. Third-party h
 - Press Enter for github.com, or enter your GitHub Enterprise Server domain
 - If you get "model not supported", enable it in VS Code: Copilot Chat → model selector → select model → "Enable"
 
+### xAI (Grok/X subscription)
+
+- Run `/login xai`, then select **Use a subscription**
+- `XAI_API_KEY` remains available through **Use an API key**
+
 ### Radius
 
 Radius is a dynamic `pi-messages` gateway. `/login radius` stores OAuth tokens in `auth.json`; the gateway catalog is refreshed independently and cached in `models-store.json`. Custom Radius gateways can be declared in `models.json` with `"oauth": "radius"` and a gateway `baseUrl`.