opencode 1.18.14: single device-code xAI login, provider retry fixes

Collapses xAI login into one device-code flow that works headless and remote, and improves provider error handling so transient and mid-stream failures are retried instead of failing the turn. Also fixes remote-workspace path resolution and ACP usage accounting.

Tool: opencode · Version: v1.18.14 · Significance: 2/5 · fix, feature