Bring back the subscribe form on the redesigned page

[scgopi]

The landing-page rebuild in August (88cfa26) removed the Buttondown subscribe form along with the old article layout. buttondown_username in _config.yml has been wired to nothing since.

Change

• The form returns inside the closing call-to-action card, under the brew command.
• It is styled from the page's own tokens (--bg, --line4, --amber) and stacks to one column under the existing phone breakpoint.
• Copy is the original: Get the next one. New releases and the next article — nothing else, and one click to leave.
• It still renders only when buttondown_username is set, so merging this alone changes nothing on the live site.

Verified

Built locally with Jekyll 3.10.0, the Pages version:

Build	<form> tags	Action
_config.yml as committed, username empty	0	none, no Liquid leaks
with buttondown_username: placeholder-user	1	https://buttondown.com/api/emails/embed-subscribe/placeholder-user

To go live

Set buttondown_username: in docs/_config.yml to the account's username, in this PR or a follow-up.