Retain user instructions in Guardian context (#42844)

## Why

Guardian reviews need the original user instructions even when compaction or
transcript selection removes them from the active conversation window.

## What changed

- Capture complete user messages in host-owned retained context and include them
  as ordered authorization evidence alongside verified answers.
- Omit oversized instructions atomically and emit an explicit incomplete-evidence
  notice instead of exposing a potentially misleading partial permission.
- Mark user messages inherited by forked agents so they remain model-visible
  without becoming child-local authorization.

## Testing

- Cover retained instructions across compaction, resume, rollback, and agent
  forks, including oversized evidence and legacy context behavior.

GitOrigin-RevId: 97122509d818c90d2f87c6a1b29c99c05ad0316e

codex-rs/guardian-context/src/lib.rs

@@ -16,6 +16,7 @@ use codex_protocol::models::ResponseItem;
 
 use authorization::RootConversationSection;
 use authorization::TrustedUserAnswersSection;
+use retained_instructions::RetainedUserInstructionsSection;
 use transcript::ConversationTranscriptSection;
 
 pub use authorization::GuardianRootMessage;
@@ -40,6 +41,8 @@ pub use verified_answers::RenderedVerifiedAnswers;
 pub use verified_answers::render_verified_answer;
 pub use verified_answers::render_verified_answers;
 
+mod retained_instructions;
+
 mod authorization;
 mod composition;
 mod entry;
@@ -102,6 +105,11 @@ pub struct SectionInput<'a> {
 pub trait SectionHistory: Send + Sync {
     /// Returns borrowed response items in their original conversation order.
     fn items(&self) -> Box<dyn Iterator<Item = &ResponseItem> + Send + '_>;
+
+    /// Bounded host-owned facts from the same snapshot as the current items.
+    fn retained_context(&self) -> Option<&codex_history::RetainedContext> {
+        None
+    }
 }
 
 impl SectionHistory for Vec<ResponseItem> {
@@ -170,6 +178,7 @@ pub fn default_registry() -> &'static SectionRegistry {
     static REGISTRY: LazyLock<SectionRegistry> = LazyLock::new(|| {
         let mut registry = SectionRegistry::default();
         registry.register(RootConversationSection);
+        registry.register(RetainedUserInstructionsSection);
         registry.register(TrustedUserAnswersSection);
         registry.register(ConversationTranscriptSection);
         registry
@@ -213,6 +222,9 @@ pub enum ContextSection {
     TrustedUserAnswers {
         items: Vec<String>,
     },
+    RetainedUserInstructions {
+        items: Vec<String>,
+    },
 }
 
 #[cfg(test)]