feat: Phase 4 — CAS-based fork + mark-and-sweep GC

- Rewrite fork to create StateNode pointing to fork point (zero duplication)
- Rewrite GC as mark-and-sweep: roots from threads.json + history, findReachableHashes via refs[]
- Remove .data.jsonl code paths
- Fix all 7 previously failing CLI tests
- New: gc-mark-sweep.test.ts verifying shared nodes survive GC
- All 166 tests pass

Refs #155, closes #159

小橘 <xiaoju@shazhou.work>

packages/workflow-execute/__tests__/engine.test.ts

@@ -40,6 +40,8 @@ function makeOptions(overrides: Partial<ExecuteThreadOptions>): ExecuteThreadOpt
     awaitAfterEachYield: async () => {},
     forkSourceThreadId: null,
     prefilledDiskSteps: null,
+    forkContinuation: null,
+    replayTimestamps: null,
     storageRoot: "/tmp/never",
     ...overrides,
   };