container: Add Pi coding agent (pi.dev) as a third harness

- Install @earendil-works/pi-coding-agent globally via npm in the
  image so it survives the home directory bind mount
- Add pi subcommand forwarding PI_* and provider key prefixes
- Add pi shell wrapper alias
- Build with --network host so image builds work on hosts where the
  Docker bridge network cannot create veth pairs

agent.aliases

@@ -12,6 +12,10 @@ claude() {
     python3 "$AGENT_CONTAINER_DIR/agent-container.py" claude "$@"
 }
 
+pi() {
+    python3 "$AGENT_CONTAINER_DIR/agent-container.py" pi "$@"
+}
+
 agent-container() {
     python3 "$AGENT_CONTAINER_DIR/agent-container.py" "$@"
 }