Require camera name explicitly across all seasons scripts

No script defaults to SUNRISE_CAM (east) for the seasons pipeline.
Every direction is treated equally — camera name is a required arg,
not an optional override.

- 4-seasons.sh: $1 required, exits with usage if missing
- montage-mvt.sh: arg order changed to <cam> [date]; $1 required
- year-end-join.sh: $2 (cam) required
- verify-mvt.sh: $4 (cam) required
- migrate-seasons.sh: example commands updated to new arg order
- README.md: montage-mvt.sh examples updated

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

year-end-join.sh

@@ -20,8 +20,11 @@ if [ -z "$ASTRO_YEAR" ]; then
     echo "Usage: $0 <ASTRO_YEAR> [CAM_NAME]  (e.g. $0 2025 north)"
     exit 1
 fi
-# Camera name: second argument overrides conf (passed through from montage-mvt.sh).
-CAM_NAME="${2:-$SUNRISE_CAM}"
+CAM_NAME="${2:-}"
+if [ -z "$CAM_NAME" ]; then
+    echo "Usage: $0 <ASTRO_YEAR> <CAM_NAME>  (e.g. $0 2025 north)"
+    exit 1
+fi
 
 year_dir="$MOVIES_DIR/$CAM_NAME/$ASTRO_YEAR"