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.

scripts/build.ts

@@ -0,0 +1,29 @@
+import { execFileSync } from 'node:child_process'
+import { existsSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+// Bundle JS for every workspace package with dumble (tsc -b has already
+// produced the .d.ts files; dumble reads each package's tsconfig/package.json).
+const packages = [
+  'vendor/cosmokit',
+  'vendor/schemastery',
+  'vendor/cordis',
+  'vendor/loader',
+  'vendor/include',
+  'vendor/group',
+  'vendor/timer',
+  'vendor/hmr',
+  'vendor/logger-console',
+  'packages/llm',
+  'packages/session',
+  'packages/system-prompt',
+  'packages/tools',
+  'packages/agent',
+  'packages/agent-loop',
+]
+
+const root = resolve(import.meta.dirname, '..')
+for (const path of packages) {
+  if (!existsSync(resolve(root, path, 'tsconfig.json'))) continue
+  execFileSync('node_modules/.bin/dumble', [path], { cwd: root, stdio: 'inherit' })
+}