Improve goal continuation based on feedback (#22045)

## Summary

This PR updates the goal continuation prompt to address feedback from
early adopters. There are two primary changes:

1. Goal continuation and budget-limit steering prompts now use hidden
user-context messages instead of hidden developer messages.
2. The goal continuation prompt is refined to improve the model's
ability to fully complete the active goal rather than stop at a smaller
or merely passing subset.

The user-message transition is important for two reasons. First, it
eliminates an issue where older steering messages could be responded to
again after a new turn. Second, it works better with compaction because
user messages are treated differently from developer messages during
compaction.

The prompt refinements make persistence explicit, ground work in current
evidence, encourage `update_plan` for multi-step progress visibility,
and require stronger completion audits before calling `update_goal`. It
also removes the elapsed-time reporting in the prompt; I saw evidence
that this was causing the model to shortcut work as it became nervous
about time.

These changes were tested with evals. Chriss4123 has also been running
independent evals in
[#19910](https://github.com/openai/codex/issues/19910), and many of the
improvements in this PR were suggested by him.

## Verification

- Tested with evals.
- Added and updated focused `codex-core` coverage for hidden goal user
context, continuation and budget-limit request shape, prompt rendering,
and objective delimiter escaping.

codex-rs/core/src/context/contextual_user_message.rs

@@ -5,6 +5,7 @@ use codex_protocol::models::ContentItem;
 use super::EnvironmentContext;
 use super::FragmentRegistration;
 use super::FragmentRegistrationProxy;
+use super::GoalContext;
 use super::SkillInstructions;
 use super::SubagentNotification;
 use super::TurnAborted;
@@ -23,6 +24,8 @@ static TURN_ABORTED_REGISTRATION: FragmentRegistrationProxy<TurnAborted> =
     FragmentRegistrationProxy::new();
 static SUBAGENT_NOTIFICATION_REGISTRATION: FragmentRegistrationProxy<SubagentNotification> =
     FragmentRegistrationProxy::new();
+static GOAL_CONTEXT_REGISTRATION: FragmentRegistrationProxy<GoalContext> =
+    FragmentRegistrationProxy::new();
 
 static CONTEXTUAL_USER_FRAGMENTS: &[&dyn FragmentRegistration] = &[
     &USER_INSTRUCTIONS_REGISTRATION,
@@ -31,6 +34,7 @@ static CONTEXTUAL_USER_FRAGMENTS: &[&dyn FragmentRegistration] = &[
     &USER_SHELL_COMMAND_REGISTRATION,
     &TURN_ABORTED_REGISTRATION,
     &SUBAGENT_NOTIFICATION_REGISTRATION,
+    &GOAL_CONTEXT_REGISTRATION,
 ];
 
 fn is_standard_contextual_user_text(text: &str) -> bool {

codex-rs/core/src/context/contextual_user_message_tests.rs

@@ -26,6 +26,18 @@ fn detects_subagent_notification_fragment_case_insensitively() {
     ));
 }
 
+#[test]
+fn detects_goal_context_fragment() {
+    let text = GoalContext {
+        prompt: "Continue working toward the active thread goal.".to_string(),
+    }
+    .render();
+
+    assert!(is_contextual_user_fragment(&ContentItem::InputText {
+        text
+    }));
+}
+
 #[test]
 fn ignores_regular_user_text() {
     assert!(!is_contextual_user_fragment(&ContentItem::InputText {

codex-rs/core/src/context/goal_context.rs

@@ -0,0 +1,18 @@
+//! Hidden user-context fragment for runtime-owned goal steering prompts.
+
+use super::ContextualUserFragment;
+
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct GoalContext {
+    pub(crate) prompt: String,
+}
+
+impl ContextualUserFragment for GoalContext {
+    const ROLE: &'static str = "user";
+    const START_MARKER: &'static str = "<goal_context>";
+    const END_MARKER: &'static str = "</goal_context>";
+
+    fn body(&self) -> String {
+        format!("\n{}\n", self.prompt)
+    }
+}

codex-rs/core/src/context/mod.rs

@@ -8,6 +8,7 @@ mod collaboration_mode_instructions;
 mod contextual_user_message;
 mod environment_context;
 mod fragment;
+mod goal_context;
 mod guardian_followup_review_reminder;
 mod hook_additional_context;
 mod image_generation_instructions;
@@ -36,6 +37,7 @@ pub(crate) use environment_context::EnvironmentContext;
 pub use fragment::ContextualUserFragment;
 pub(crate) use fragment::FragmentRegistration;
 pub(crate) use fragment::FragmentRegistrationProxy;
+pub(crate) use goal_context::GoalContext;
 pub(crate) use guardian_followup_review_reminder::GuardianFollowupReviewReminder;
 pub(crate) use hook_additional_context::HookAdditionalContext;
 pub(crate) use image_generation_instructions::ImageGenerationInstructions;