feat: polluted memories (#13008)

Add a feature flag to disable memory creation for "polluted"

codex-rs/state/src/model/thread_metadata.rs

@@ -45,6 +45,8 @@ pub struct ThreadsPage {
 pub struct ExtractionOutcome {
     /// The extracted thread metadata.
     pub metadata: ThreadMetadata,
+    /// The explicit thread memory mode from rollout metadata, if present.
+    pub memory_mode: Option<String>,
     /// The number of rollout lines that failed to parse.
     pub parse_errors: usize,
 }