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Delete what the ROS design superseded: the Picamera2 MCP server and its unit, the old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-config-snippet.yaml

@@ -0,0 +1,24 @@
+# Merge these into ~/.hermes/config.yaml on Omarchy.
+# The robot is at 10.55.0.2 on the USB link (the host is 10.55.0.1; see host/omarchy/robot-usb-link.sh).
+# Put the token in ~/.hermes/.env as:   ROBOT_MCP_TOKEN=<64 hex characters>
+# It lives only on the Pi, in /etc/robot-mcp.env (read it with: ssh hermes@10.55.0.2 sudo cat /etc/robot-mcp.env).
+# The robot's firewall accepts this port only over the USB link, from 10.55.0.1.
+# While Hermes holds a connection the robot's face is awake; when it stops, the face falls asleep after
+# about 40 seconds. A robot with no head fitted offers only look(); turn_head appears when servos do.
+
+mcp_servers:
+  robot:
+    url: "http://10.55.0.2:8765/mcp"
+    headers:
+      Authorization: "Bearer ${ROBOT_MCP_TOKEN}"
+    connect_timeout: 15
+    timeout: 30          # look() normally answers in about a second
+
+voice:
+  auto_tts: true         # speak replies whenever voice mode is on
+  # The robot's mic sits beside its own speaker. If Hermes interrupts itself,
+  # raise this (default 3.0) or enable the Pi's optional echo-cancel config.
+  barge_in_threshold_multiplier: 4.0
+  # If the level bar barely moves when you speak, lower this (default 200)
+  # or raise the mic volume on the Pi (see README).
+  silence_threshold: 200