fix: address review findings — CJS build (tsup), workflowCost signature, bottlenecks empty-graph test

- C1(critical): Replace tsc build with tsup for dual ESM + CJS output
- W2(warning): Change workflowCost to accept TaskGraph instead of TaskGraphInner
- S1(suggestion): Add test for bottlenecks empty-graph early return
- S2(suggestion): Document dangling-reference detection is unreachable via public API

tsup.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'tsup';
+
+export default defineConfig({
+  entry: ['src/index.ts'],
+  format: ['esm', 'cjs'],
+  dts: true,
+  sourcemap: true,
+  clean: true,
+  target: 'es2022',
+});