feat(ai): adopt upstream Grok subscription authentication in Prime

[rynfar]

Prime upstream adds Grok 4.5 subscription login and automatic token refresh while preserving xAI API-key access.

Review and adopt Prime upstream #2252, merged as ca67580. This enables Grok through Prime's xAI model provider; Pylon's separate Grok provider is a different integration.

Acceptance:

• Verify subscription login, token refresh, expired/revoked credential handling, and continued API-key access.
• Preserve correct authentication and request routing when switching authentication methods or models in an existing Prime session.
• Run the upstream mocked xAI OAuth/request regressions and affected model-registry, SDK, in-process, and daemon connection tests.
• Verify Pylon's native Prime authentication status/model discovery reflect the available access and preserve owned-session continuity.
• Document the subscription setup through Prime and any supported-model limitations.

Suggested priority: feature follow-up after the reliability/authentication fixes.

Backlog follow-up under #1 and Pylon's Prime integration tracker. The completed September integration remains the baseline; this issue tracks later upstream adoption. Before implementation, refresh upstream evidence and record the ownership decision and focused verification in .pylon/features.yaml and .pylon/upstream-review.md.