Add e2e substrate tests with mocks.

[wjjclaud]

Summary

• Build and test with -tags harness in CI so the substrate/harness deployment path is actually compiled and exercised on every PR.
• Add a hermetic end-to-end test for SubstrateHarness (Start → Run → Close) using in-process fakes for the substrate control plane and the actor's HarnessService.
• Extract shared test mocks into mocks_test.go and move the mocks from antigravity_test.go onto the shared file, including:
  • A fakeControlServer for substrate control plane
  • A fakeHarnessServer for a harness service