app-server: Add ephemeral field to Thread object (#13084)

Currently there is no alternative way to know that thread is ephemeral,
only client which did create it has the knowledge.

codex-rs/app-server-protocol/schema/json/v2/ThreadReadResponse.json

@@ -655,6 +655,10 @@
           "description": "Working directory captured for the thread.",
           "type": "string"
         },
+        "ephemeral": {
+          "description": "Whether the thread is ephemeral and should not be materialized on disk.",
+          "type": "boolean"
+        },
         "gitInfo": {
           "anyOf": [
             {
@@ -724,6 +728,7 @@
         "cliVersion",
         "createdAt",
         "cwd",
+        "ephemeral",
         "id",
         "modelProvider",
         "preview",