feat(P0-001): scaffold monorepo structure

pnpm workspaces, Turborepo, TypeScript strict, ESLint flat config,
Prettier, Vitest, husky + lint-staged. Placeholder packages for all
16 packages + 2 apps + 2 plugins.

Closes #1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vitest.workspace.ts

@@ -0,0 +1,7 @@
+import { defineWorkspace } from 'vitest/config';
+
+export default defineWorkspace([
+  'packages/*/vitest.config.ts',
+  'apps/*/vitest.config.ts',
+  'plugins/*/vitest.config.ts',
+]);