feat(llm): opt-in Grok/SuperGrok subscription OAuth

Add Sign in with Grok mirroring the merged ChatGPT/Codex integration: a strix/config/grok.py OAuth module (PKCE loopback flow against auth.x.ai, refresh with cross-process locking, secure ~/.strix/subscription-auth.json store) plus grok/<model> routing through the OpenAI-compatible api.x.ai/v1 endpoint via a bearer-stamping chat-completions client.

strix auth login grok / status / logout become provider-aware; subscription.py shares provider-agnostic auth-mode detection; Grok runs are marked zero-cost like other subscriptions.

Uses a consumer subscription outside xAI own products, which xAI does not officially support; opt-in and experimental.

pyproject.toml

@@ -217,6 +217,7 @@ ignore = [
 # args they intentionally ignore.
 "tests/test_viewer_auth.py" = ["S105", "S106", "ARG001"]
 "tests/test_codex_auth.py" = ["S105", "S106", "SLF001"]
+"tests/test_grok_auth.py" = ["S105", "S106", "SLF001"]
 # Stdlib HTTP handler overrides (do_GET/do_POST).
 "strix/interface/auth_cli.py" = ["N802"]
 "tests/test_codex_streaming.py" = ["N802"]
@@ -268,6 +269,7 @@ ignore = [
 # Heavy inference deps (httpx, openai) imported lazily so auth-status checks
 # don't pull them in.
 "strix/config/codex.py" = ["PLC0415"]
+"strix/config/grok.py" = ["PLC0415"]
 # Interface utility branches per scope-mode / target-type combination;
 # splitting would obscure the decision tree without simplifying it.
 "strix/interface/utils.py" = ["PLR0912", "BLE001", "PLC0415"]