fix(agents): rename xai provider key to xai-oc to avoid opencode catalog collision (#118)

opencode ships a built-in `xai` model-catalog entry (a real @ai-sdk/xai
integration with Responses-API support). Naming our custom OpenAI-compatible
shim `xai` too made opencode's --auto permission-check path assume the
catalog's provider object once a `permission` block is present (always, in
this pipeline) and call .responses(), which our shim doesn't implement —
crashing every @lead/@ops run with "Z.responses is not a function".

Reproduced locally with the production config shape on opencode 1.17.13;
renaming the provider key to xai-oc (model id xai-oc/grok-4.5) fixes it
reliably across repeated fresh-state runs.

.gitea/workflows/scripts/agents.json

@@ -25,7 +25,7 @@
     "desc": "Senior dev — complex, multi-file implementation (GLM-5.2 via Ollama Cloud, text-only)."
   },
   "lead": {
-    "model": "xai/grok-4.5",
+    "model": "xai-oc/grok-4.5",
     "vision": true,
     "mode": "pr",
     "skills": [
@@ -43,7 +43,7 @@
     "desc": "QA / reviewer — reviews PRs: reads the diff, drives a headless browser (Playwright) to verify behavior, posts specific recommendations on the PR and the pass/fail verdict on the issue. Never edits code, never merges."
   },
   "ops": {
-    "model": "xai/grok-4.5",
+    "model": "xai-oc/grok-4.5",
     "vision": false,
     "mode": "comment",
     "skills": [