docs: image brief and treatment tooling

- docs/image-brief.md: one treatment for every image, keyed to the site palette
- tools/images/treat.py: cutout (rembg) and treat (Pillow) as separate, re-runnable steps
- tools/images/README.md: how to run it without polluting the NixOS python

tools/images/README.md

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+# Image treatment
+
+Implements `docs/image-brief.md`. Two steps, in this order.
+
+## 1. Cut the subject out
+
+Needs `rembg`. It is not installed on the host, and installing Python packages on NixOS
+into the system interpreter is a bad idea. Use a container:
+
+```
+docker run --rm -v "$PWD/raw:/in" -v "$PWD/build/cut:/out" \
+  -v "$PWD/tools/images/treat.py:/treat.py" \
+  python:3.12-slim sh -c "pip install -q rembg[cpu] pillow onnxruntime && \
+  python /treat.py cutout /in/*.jpg --out /out"
+```
+
+Or cut out by hand in GIMP and skip to step 2. For fewer than ten images, hand-cutting is
+usually better — automatic segmentation struggles with reflective or thin edges.
+
+## 2. Apply the treatment
+
+```
+pip install pillow        # or the same container, without rembg
+python3 tools/images/treat.py treat build/cut/*.png --size hero
+```
+
+Output lands in `content/assets/images/` as WebP at quality 82, canvas `#0b0d10`, subject
+right of centre, left-to-right fade, rim glow from the alpha channel.
+
+## Then
+
+1. Add the page reference: `![caption](../../assets/images/name.webp)`
+2. File the original in the project's own repo under `docs/images/`, with a note in that
+   folder's `README.md` saying where it came from.
+3. Deploy and check it on the live site at 1440×900 and on a phone.