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.build.json

@@ -0,0 +1,20 @@
+{
+  "files": [],
+  "references": [
+    { "path": "./vendor/cosmokit" },
+    { "path": "./vendor/schemastery" },
+    { "path": "./vendor/cordis" },
+    { "path": "./vendor/loader" },
+    { "path": "./vendor/include" },
+    { "path": "./vendor/group" },
+    { "path": "./vendor/timer" },
+    { "path": "./vendor/hmr" },
+    { "path": "./vendor/logger-console" },
+    { "path": "./packages/llm" },
+    { "path": "./packages/session" },
+    { "path": "./packages/system-prompt" },
+    { "path": "./packages/agent" },
+    { "path": "./packages/tools" },
+    { "path": "./packages/agent-loop" }
+  ]
+}