Add a camera calibration tool and a checkerboard to show on a screen

hermes_vision.calibrate collects 20 varied checkerboard views from the camera topic
(rejecting repeats of a pose, which constrain nothing), solves with OpenCV and writes
the camera_info YAML camera_ros loads, named with the resolution as it expects.
Tested: the solver recovers a known synthetic camera (focal length within 1%), the
YAML round-trips, and real board detection works on a rendered board. Not yet run on
the real camera: it needs someone holding a board in front of it, with the focus set.

docs/checkerboard-9x6.png is a 10x7-square board that can be shown on a screen.

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

README.md

@@ -130,6 +130,22 @@ give MCP images to the main model: it saves the photo to its image cache and the
 - **Run a node by hand:** `source ~/robot-env.sh` first: it puts the libcamera fork and the right
   OpenCV on the path, and keeps ROS local.
 
+## Calibrating the camera
+
+Needed once, and again if you change the lens focus or the image size. Set the focus first (the lens
+turns if it is not glued: focus at the distance you will use), then show `docs/checkerboard-9x6.png`
+on a screen (or print it) and measure one square with a ruler. With the robot running, on the Pi, after
+`source ~/robot-env.sh`:
+
+```
+python3 -m hermes_vision.calibrate --cols 9 --rows 6 --square-mm <measured>
+```
+
+Hold the camera 25 to 60 cm from the board and vary distance, tilt and position, keeping all of it in
+view. It collects 20 different views, solves with OpenCV and writes
+`~/.ros/camera_info/ov5647__base_soc_i2c0mux_i2c_1_ov5647_36_1296x972.yaml`. A reprojection error under
+about 0.5 px is good. `sudo systemctl restart robot` to load it.
+
 ## Tuning and troubleshooting
 
 - **Recordings are exactly silent:** the Audio switch is off (see above).