feat(agentci): add gemini runner backend

Support gemini -p -y (non-interactive yolo mode) alongside claude
and codex runners. Three AI backends for different cost profiles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/agent-runner.sh

@@ -114,6 +114,14 @@ case "$RUNNER" in
             "$PROMPT" \
             > "$LOG_FILE" 2>&1
         ;;
+    gemini)
+        MODEL_FLAG=""
+        if [ -n "$MODEL" ] && [ "$MODEL" != "sonnet" ]; then
+            MODEL_FLAG="-m $MODEL"
+        fi
+        echo "$PROMPT" | gemini -p - -y $MODEL_FLAG \
+            > "$LOG_FILE" 2>&1
+        ;;
     *)
         echo "$PROMPT" | claude -p \
             --model "$MODEL" \