chore: switch @lead/@ops from Anthropic to xAI grok-4.5

Removes the last Anthropic-model usage from the agent registry.
XAI_API_KEY was already provisioned as an org secret; wire it into
the run-agent step and update every doc/comment that referenced
ANTHROPIC_API_KEY or claude-opus-4-8.

.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": "anthropic/claude-opus-4-8",
+    "model": "xai/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": "anthropic/claude-opus-4-8",
+    "model": "xai/grok-4.5",
     "vision": false,
     "mode": "comment",
     "skills": [