feat(app): define trusted rewind branch flow

contracts/schema/fork-branch-request.schema.json

@@ -0,0 +1,30 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "ForkBranchRequest",
+  "description": "Requests a new playable branch rooted at an already committed story node.\n\nThe caller identifies the branch head it observed so the trusted runtime can\nreject a rewind racing with a newer turn. The new branch id is generated by\nthe runtime from these trusted inputs and is never supplied by the UI or the\nmodel.",
+  "type": "object",
+  "properties": {
+    "actionId": {
+      "type": "string"
+    },
+    "currentBranchId": {
+      "type": "string"
+    },
+    "expectedCurrentNodeId": {
+      "type": "string"
+    },
+    "sourceNodeId": {
+      "type": "string"
+    },
+    "storyId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "storyId",
+    "currentBranchId",
+    "expectedCurrentNodeId",
+    "sourceNodeId",
+    "actionId"
+  ]
+}