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.

tsconfig.base.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "es2024",
+    "module": "esnext",
+    "moduleResolution": "bundler",
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "composite": true,
+    "incremental": true,
+    "skipLibCheck": true,
+    "esModuleInterop": true,
+    "allowImportingTsExtensions": true,
+    "strict": true,
+    "noImplicitAny": false,
+    "types": ["node"]
+  }
+}