Clean out the Pi Zero leftovers, rewrite the README, and add the one installer

Wi-Fi-off acceptance passed: with Wi-Fi blocked, live and after a reboot, the Pi
came up on the USB link alone, every node ran, and hermes mcp test robot and look()
worked. (A temporary guard that would have unblocked Wi-Fi was the safety net and is
removed; Wi-Fi is back on.)

Delete what the ROS design superseded: the Picamera2 MCP server and its unit, the old
installer and requirements, and the duplicate platform check. Move omarchy/ to
host/omarchy/. Rewrite README.md for this build: parts, physical steps (including the
Audio switch), setup, daily use, the troubleshooting learned the hard way, and the
security model.

pi/install.sh is now the single installer for a fresh Ubuntu 24.04 image, built from the
steps done by hand: ROS, the libcamera fork at a pinned commit, camera_ros, the venv and
models, the workspace, USB gadget, audio overlay, firewall (before PipeWire opens a
port), PipeWire, mixer and services. Run twice on the live robot, the second run changed
nothing and everything stayed healthy. It has not been run on a blank card.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AFN8nRRCiDiMN7ChBpHJhs

host/omarchy/hermes-robot

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# hermes-robot: start Hermes with the robot as the default speaker and microphone,
+# then put your usual devices back when Hermes exits.
+#
+# Install:  install -Dm755 hermes-robot ~/.local/bin/hermes-robot
+# Use:      hermes-robot            (any arguments are passed through to hermes)
+set -uo pipefail
+
+SINK=robot_speaker
+SOURCE=robot_mic
+
+has() { pactl list short "$1" | awk '{print $2}' | grep -qxF "$2"; }
+if ! has sinks "$SINK" || ! has sources "$SOURCE"; then
+    echo "hermes-robot: the robot's audio devices aren't present." >&2
+    echo "  Is the robot powered and plugged in over USB (ping 10.55.0.2)? Check with:  pactl list short sources" >&2
+    exit 1
+fi
+
+prev_sink="$(pactl get-default-sink)"
+prev_source="$(pactl get-default-source)"
+restore() {
+    pactl set-default-sink "$prev_sink" 2>/dev/null
+    pactl set-default-source "$prev_source" 2>/dev/null
+}
+trap restore EXIT INT TERM
+
+pactl set-default-sink "$SINK"
+pactl set-default-source "$SOURCE"
+
+hermes "$@"