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/tokens/utilities.css

@@ -2,7 +2,7 @@
  * The token utilities, for the components still written in Tailwind.
  *
  * Tailwind's side of tokens/state.css and tokens/type.css, kept apart so that the token files the
- * foundation imports carry no Tailwind directive. material.css imports this file after Tailwind;
+ * foundation imports carry no Tailwind directive. tailwind.css imports this file after Tailwind;
  * it goes when the last component leaves Tailwind. New markup uses the plain-CSS classes instead,
  * which carry the same declarations: `md-state-layer`, `md-focus-ring`, `md-touch-target` and
  * `md-link` (foundation/interaction.css), and `md-type-*` (text.css).