Add Push MD demo remote

[adamziel]

What it does

Adds the remaining demo pieces needed for pushmd.blog without replacing the current landing-page logo or social-card assets:

• working terminal tabs for the landing-page emulator
• a public read-only static Git remote at /wp-json/git/v1/md.git
• a landing-page clone command that points at https://pushmd.blog/wp-json/git/v1/md.git
• install-section copy that lets visitors try the pullable public remote first, then install Push MD on their own WordPress site when they are ready to accept pushes
• small responsive fixes for the landing page controls and mobile overflow

Rationale

The landing page should give people an immediate pullable Git remote before they install the plugin on their own WordPress site. The public demo remote stays read-only; authenticated Push MD installs remain the path for accepting writes through WordPress permissions.

Implementation

The static Git remote is checked into the GitHub Pages artifact path under plugins/push-md/docs/landing/wp-json/git/v1/md.git, with server-info generated so dumb HTTP cloning works from static hosting.

The terminal emulator now switches between Markdown, permissions/write, and pull/admin-edit demos. It also makes the public remote's read-only behavior explicit when a user tries to push.

Testing instructions

Validated locally with:

git diff --check
vendor/bin/phpcs -d memory_limit=1G plugins/push-md/docs/landing
node --check plugins/push-md/docs/landing/landing.js

Confirmed the static remote works over HTTP:

python3 -m http.server 45905 --bind 127.0.0.1 --directory plugins/push-md/docs/landing
git clone http://127.0.0.1:45905/wp-json/git/v1/md.git /tmp/my-site
git -C /tmp/my-site pull --ff-only