fix(pi-runner): allow host-side extension path override for task runs

Task-run pi launches hardcoded the container path /app/extensions/tavily-search.mjs;
on the no-docker host sidecar every squad/dialogue task run failed at spawn with
ENOENT/extension-load errors. Add PI_AGENT_EXTENSION override and export the
managed pi-dir extension path from the sidecar start script.

deploy/pi-agent-sidecar/start.sh

@@ -76,6 +76,7 @@ cat > "${PI_DIR}/settings.json" <<EOF
 EOF
 
 export PORT DATA_DIR PI_CODING_AGENT_DIR="$PI_DIR"
+export PI_AGENT_EXTENSION="${PI_DIR}/extensions/tavily-search.mjs"
 export PATH="${PATH}:/home/linuxbrew/.linuxbrew/bin"
 
 echo "================================================================="