feat(FN-4222): add experiment-finalize CLI command and API route

Implements the `experiment-finalize` CLI command (FN-4222) and its companion dashboard API route, wiring the feature through the pi extension as a new callable tool. Includes the command implementation, extension tooling, API integration, and corresponding test coverage.

Fusion-Task-Id: FN-4222

packages/engine/src/__tests__/experiment-executor.test.ts

@@ -21,6 +21,13 @@ function createGitMock(): GitOps {
     stashPush: vi.fn(),
     stashPop: vi.fn(),
     statusPorcelain: vi.fn(),
+    mergeBase: vi.fn(),
+    branchExists: vi.fn(),
+    createBranch: vi.fn(),
+    cherryPick: vi.fn(),
+    checkout: vi.fn(),
+    currentBranch: vi.fn(),
+    deleteBranch: vi.fn(),
   };
 }
 

packages/engine/src/__tests__/experiment-git-policy.test.ts

@@ -53,6 +53,13 @@ function createGitMock(): GitOps {
     stashPush: vi.fn(),
     stashPop: vi.fn(),
     statusPorcelain: vi.fn(),
+    mergeBase: vi.fn(),
+    branchExists: vi.fn(),
+    createBranch: vi.fn(),
+    cherryPick: vi.fn(),
+    checkout: vi.fn(),
+    currentBranch: vi.fn(),
+    deleteBranch: vi.fn(),
   };
 }