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

Dockerfile

@@ -37,4 +37,10 @@ RUN curl -fsSL https://claude.ai/install.sh | bash && \
     rm -rf ~/.local/share/claude ~/.local/bin/claude ~/.claude ~/.claude.json \
            ~/.cache/claude
 
+# Install Pi Coding Agent globally with npm so it lands in the system
+# prefix and survives the home directory bind mount (same approach as
+# pi's own docs/containerization.md)
+RUN sudo npm install -g --ignore-scripts @earendil-works/pi-coding-agent && \
+    sudo npm cache clean --force
+
 WORKDIR /home/${USERNAME}