Remove material.css, the 1.x single-import shortcut

Plan step 37: an application on 1.x-style imports now imports
foundation.css before @import 'tailwindcss' and tailwind.css after it,
the way foundation.css's own header has described since Phase F
started — material.css only ever saved writing those two lines.
Updates every place that named it: tailwind.css's and
foundation/hidden.css's and tokens/utilities.css's header comments,
the development skill's install snippet and Tokens section, README.md's
install snippet, and a new line in UPGRADE.md's migration notes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/foundation/hidden.css

@@ -7,9 +7,10 @@
  * `hidden="until-found"` is left to the browser, which keeps such content searchable.
  * `[x-cloak]` hides what Alpine has not started yet; Alpine removes the attribute when it has.
  *
- * Only a layered `!important` could outrank these, and the package writes none. The file is
- * apart from reset.css because material.css, which stays for applications importing it after
- * Tailwind, needs `[x-cloak]` without the reset.
+ * Only a layered `!important` could outrank these, and the package writes none. The file is apart
+ * from reset.css because these two rules are unlayered while reset.css's are in `material.reset`;
+ * foundation.css is the only file that imports it, so every entry — `all.css` and an application's
+ * own — reaches it once, through the foundation.
  */
 
 @layer material.reset, material.tokens, material.base, material.layout, material.components, material.text, material.visibility;