feat: ephemeral threads (#9765)

Add ephemeral threads capabilities. Only exposed through the
`app-server` v2

The idea is to disable the rollout recorder for those threads.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -1086,6 +1086,7 @@ pub struct ThreadStartParams {
     pub base_instructions: Option<String>,
     pub developer_instructions: Option<String>,
     pub personality: Option<Personality>,
+    pub ephemeral: Option<bool>,
     /// If true, opt into emitting raw response items on the event stream.
     ///
     /// This is for internal use only (e.g. Codex Cloud).
@@ -1469,7 +1470,7 @@ pub struct Thread {
     #[ts(type = "number")]
     pub updated_at: i64,
     /// [UNSTABLE] Path to the thread on disk.
-    pub path: PathBuf,
+    pub path: Option<PathBuf>,
     /// Working directory captured for the thread.
     pub cwd: PathBuf,
     /// Version of the CLI that created the thread.