Add hermes_mcp, the bridge between Hermes and the ROS graph

The MCP server is now a ROS node and the only way in or out, per the owner's
decision that ROS never leaves the robot. It serves MCP on the USB address only
with a bearer token, keeps the stable tool name look() (which waits for the next
camera frame, so it is never stale, and passes the camera's JPEG through), and
registers turn_head/head_position only when a head is fitted, which today it is
not. While an authenticated client is connected it republishes the agent
heartbeat on /hermes/state, so the face wakes on connect and sleeps about 40 s
after the client leaves; look() also makes the face squint.

The token is generated on the Pi into /etc/robot-mcp.env (root:hermes, 640) and
never printed. Verified end to end from talos with a real MCP client over the USB
link: tools listed, a 157 KB JPEG back in 0.08 s, a wrong token refused, the Wi-Fi
path dropped by the firewall. Host config snippets now point at 10.55.0.2.

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

ros/hermes_bringup/config/robot.yaml

@@ -30,3 +30,11 @@ hermes_face_detector:
 hermes_audio_state:
   ros__parameters:
     rate_hz: 30.0
+
+hermes_mcp:
+  ros__parameters:
+    bind: 10.55.0.2                    # the USB link; the firewall accepts 8765 only from 10.55.0.1 on usb0
+    port: 8765
+    head: false                        # no servos fitted yet: turn_head and head_position stay unregistered
+    connected_grace_s: 30.0            # an authenticated client heard within this long keeps the heartbeat going
+    look_timeout_s: 3.0