feat: add minimal run engine

tests/fixtures/fake-artifacts/dev/phase-plan@1/ok.json

@@ -0,0 +1,24 @@
+{
+  "phases": [
+    {
+      "key": "implement",
+      "title": "Implement requested change",
+      "objective": "Use the fake development run to prove the engine path.",
+      "roles": ["spec_writer"],
+      "expectedArtifact": {
+        "path": "artifacts/implementation-spec.json",
+        "schema": "dev/spec@1"
+      },
+      "gates": [],
+      "tasks": [
+        {
+          "id": "TASK-1",
+          "title": "Apply implementation",
+          "role": "spec_writer",
+          "writeSet": ["src/**"],
+          "dependsOn": []
+        }
+      ]
+    }
+  ]
+}