[codex] Add Push MD landing beta and plugin links

[artpi]

Summary

• add favicon and Apple touch icon links to the Push MD landing page using the existing logo asset
• add beta messaging to the landing page header and hero
• add the Push MD Plugin URI plus a plugin-list Landing page action link

Validation

• vendor/bin/phpcs -d memory_limit=1G plugins/push-md/push-md.php
• php -l plugins/push-md/push-md.php
• git diff --check
• local browser preview at http://127.0.0.1:8876/ confirmed beta copy, favicon links, terminal content, and no console errors
• vendor/bin/phpunit plugins/push-md/Tests/ ran, but all 5 E2E tests skipped because PUSH_MD_E2E_* env vars were not set

Notes

Full composer lint still reports existing unrelated warnings elsewhere in the repository; the touched PHP file passes targeted PHPCS.