www.yml: install ImageMagick for the mosaic step

ubuntu-24.04 runner images dropped ImageMagick entirely — the shim's
`command -v` assignment exited non-zero under `set -e` with no output.
Install on demand in the workflow step; make the script's
missing-binary failure loud.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ryan-williams

.github/workflows/www.yml

@@ -65,7 +65,11 @@ jobs:
         # gating on it would re-trigger forever. It still ships whenever a
         # PLOT screenshot changes (monthly data updates), riding the same
         # commit.
-        run: bash scripts/compose-og.sh
+        run: |
+          # ubuntu-24.04 runner images dropped ImageMagick.
+          command -v magick >/dev/null || command -v convert >/dev/null \
+            || sudo apt-get install -y -qq imagemagick
+          bash scripts/compose-og.sh
       - name: Push + re-run, if screenshots changed
         if: github.ref == 'refs/heads/www'
         id: screenshots

www/scripts/compose-og.sh

@@ -4,7 +4,11 @@
 # Requires: ImageMagick — v7 (`magick`) or v6 (`convert`, e.g. ubuntu GHA runners)
 set -e
 
-MAGICK="$(command -v magick || command -v convert)"
+MAGICK="$(command -v magick || command -v convert || true)"
+if [ -z "$MAGICK" ]; then
+  echo "compose-og: ImageMagick not found (need \`magick\` or \`convert\`)" >&2
+  exit 1
+fi
 magick() { "$MAGICK" "$@"; }
 
 cd "$(dirname "$0")/.."