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.

.gitignore

@@ -1 +1,7 @@
 CLAUDE.local.md
+node_modules/
+lib/
+*.tsbuildinfo
+.yarn/
+yarn-error.log
+examples/*/*.jsonl