fix: deploy cookie under /development/ subpath correctly

Use a trailing slash on BASE_URL so MyST joins it to root-relative
page links correctly (was producing /developmentpages/...), and bump
the cookie submodule to include the hyphenated-slug internal link fix
(scientific-python/cookie#796) so checklinks passes.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: henryiii

Makefile

@@ -12,7 +12,7 @@ prepare:
 
 cookie:
 	(cd external-content/cookie/docs && \
-	BASE_URL=/development npx --yes mystmd build --html)
+	BASE_URL=/development/ npx --yes mystmd build --html)
 	mkdir -p public/development
 	cp -r external-content/cookie/docs/_build/html/. public/development/