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/codex_app_server_protocol.schemas.json

@@ -12498,6 +12498,10 @@
             "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": [
               {
@@ -12567,6 +12571,7 @@
           "cliVersion",
           "createdAt",
           "cwd",
+          "ephemeral",
           "id",
           "modelProvider",
           "preview",