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.json

@@ -0,0 +1,23 @@
+{
+  "extends": "./tsconfig.base.json",
+  "compilerOptions": {
+    "paths": {
+      "cordis": ["./vendor/cordis/src"],
+      "cosmokit": ["./vendor/cosmokit/src"],
+      "schemastery": ["./vendor/schemastery/src"],
+      "@cordisjs/plugin-loader": ["./vendor/loader/src"],
+      "@cordisjs/plugin-include": ["./vendor/include/src"],
+      "@cordisjs/plugin-group": ["./vendor/group/src"],
+      "@cordisjs/plugin-timer": ["./vendor/timer/src"],
+      "@cordisjs/plugin-hmr": ["./vendor/hmr/src"],
+      "@cordisjs/plugin-logger-console": ["./vendor/logger-console/src"],
+      "@deepseek-ai/dsh-llm": ["./packages/llm/src"],
+      "@deepseek-ai/dsh-session": ["./packages/session/src"],
+      "@deepseek-ai/dsh-system-prompt": ["./packages/system-prompt/src"],
+      "@deepseek-ai/dsh-tools": ["./packages/tools/src"],
+      "@deepseek-ai/dsh-agent": ["./packages/agent/src"],
+      "@deepseek-ai/dsh-agent-loop": ["./packages/agent-loop/src"]
+    }
+  },
+  "files": []
+}