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/build-activity-log.sh

@@ -38,7 +38,7 @@ COST=${COST:-0}; INP=${INP:-0}; OUT=${OUT:-0}; CR=${CR:-0}; CW=${CW:-0}; RE=${RE
 IN_TOTAL=$(( INP + CR + CW ))                          # total input context processed
 # Cost label: ollama / ollama-cloud models are SUBSCRIPTION-billed (GPU-time against the plan, no
 # $/token price exists), so a "$0.0000" there would be misleading — label it a subscription instead.
-# Metered providers (anthropic/…) get the real dollar cost opencode computed.
+# Metered providers (xai/…) get the real dollar cost opencode computed.
 case "${MODEL:-}" in
   ollama*|*"/ollama"*) COSTF="subscription" ;;
   *) COSTF=$(awk -v c="$COST" 'BEGIN{printf "$%.4f", c+0}') ;;