Set up monorepo infra: Yarn 4 workspaces, tsc -b + dumble build, vitest

Workspaces are vendor/* (Cordis framework vendored as source) and
packages/* (@deepseek-ai/dsh-* harness packages). Dev/test/demo run
unbuilt via tsx + root tsconfig paths; build = tsc -b (declarations)
+ dumble (JS bundles); tests = vitest + vite-tsconfig-paths.

vitest.config.ts

@@ -0,0 +1,9 @@
+import tsconfigPaths from 'vite-tsconfig-paths'
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  plugins: [tsconfigPaths({ projects: ['./tsconfig.test.json'] })],
+  test: {
+    include: ['packages/*/tests/**/*.spec.ts'],
+  },
+})