feat: add @blinkk/root-tailwind package with rootTailwind() plugin

[stevenle]

Motivation

• Provide a first-class Tailwind integration for Root.js that wires Tailwind into the Vite/PostCSS pipeline and follows Root conventions for stylesheet entry and content globs.
• Ensure generated CSS is included via Root’s existing SSR/dev dependency injection path instead of relying on manual <link> tags.

Description

• Add a new workspace package @blinkk/root-tailwind with packaging/build metadata (package.json, tsup.config.ts, tsconfig.json, CHANGELOG.md).
• Implement rootTailwind() in packages/root-tailwind/src/core.ts that exposes options stylesheetEntry (default styles/index.css) and content globs (Root-focused defaults: routes, layouts, components, templates, elements).
• Wire Tailwind into Vite by injecting a PostCSS Tailwind plugin using tailwindcss({ content }) and add a Vite plugin that auto-injects import '/styles/index.css' into route modules so Root will include the compiled CSS through its existing asset dependency graph.
• Export DEFAULT_STYLESHEET_CONTENT containing the default Tailwind directives and add README.md showing pnpm install instructions, one-line root.config.ts usage, and migration steps from manual <link rel="stylesheet">.
• Add a changeset for the new package and include @blinkk/root-tailwind in the monorepo fixed changeset group (.changeset/bright-planets-smell.md and updated .changeset/config.json).

Testing

• Attempted pnpm install --lockfile-only to refresh lock metadata, but it failed due to environment/corepack network issues (proxy returned 403), so package manager actions were not completed.
• No unit or integration tests were executed in this rollout.

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Codex Task