Add backend remember-context RPC

Load source rollouts, extract visible conversation, persist a hidden remembered-context packet, and expose the v2 thread/remember RPC for UI integration.

Co-authored-by: Codex <noreply@openai.com>

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

@@ -0,0 +1,20 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "contextPreview": {
+      "type": "string"
+    },
+    "rememberedThreadIds": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "contextPreview",
+    "rememberedThreadIds"
+  ],
+  "title": "ThreadRememberResponse",
+  "type": "object"
+}