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

package.json

@@ -22,7 +22,8 @@
     "dist"
   ],
   "scripts": {
-    "build": "tsc",
+    "build": "tsup",
+    "build:tsc": "tsc",
     "test": "vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
@@ -49,6 +50,7 @@
   "devDependencies": {
     "@types/node": "^22.0.0",
     "@vitest/coverage-v8": "^3.2.4",
+    "tsup": "^8.5.1",
     "typescript": "^5.7.0",
     "vitest": "^3.1.0"
   },