core: add configurable <context_window_guidance> message (#29936)

## Why

This PR adds a configurable `<context_window_guidance>` developer
section immediately after `<context_window>`. Harness integrations need
this section to give the model deployment-specific instructions for
preparing for context-window transitions.

## What changed

- Add an optional `features.token_budget.guidance_message` config with a
1,000-byte runtime cap and generated schema support.
- Render configured guidance as a developer `ContextualUserFragment`
wrapped in `<context_window_guidance>` immediately after
`<context_window>`.
- Omit the section when guidance is unset, empty, or whitespace-only.
- Preserve the resolved value in config locks and classify persisted
guidance as contextual developer content.
- Add integration coverage for rendered content and ordering.

codex-rs/core/src/config/config_tests.rs

@@ -475,10 +475,12 @@ async fn load_config_resolves_token_budget_config() -> std::io::Result<()> {
 enabled = true
 reminder_threshold_tokens = 16000
 reminder_message_template = "Custom reminder: {n_remaining} tokens."
+guidance_message = "Preserve important state before compaction."
 "#,
             TokenBudgetConfig {
                 reminder_threshold_tokens: Some(16_000),
                 reminder_message_template: "Custom reminder: {n_remaining} tokens.".to_string(),
+                guidance_message: Some("Preserve important state before compaction.".to_string()),
             },
         ),
     ] {

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

@@ -1089,11 +1089,13 @@ pub(crate) const DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE: &str = concat!(
     "Once reset, message items in current context window will be cleared in the new window, but notes and history items will be persistent across windows."
 );
 const TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE_MAX_BYTES: usize = 1000;
+const TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES: usize = 1000;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct TokenBudgetConfig {
     pub reminder_threshold_tokens: Option<i64>,
     pub reminder_message_template: String,
+    pub guidance_message: Option<String>,
 }
 
 impl Default for TokenBudgetConfig {
@@ -1101,6 +1103,7 @@ impl Default for TokenBudgetConfig {
         Self {
             reminder_threshold_tokens: None,
             reminder_message_template: DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE.to_string(),
+            guidance_message: None,
         }
     }
 }
@@ -2578,9 +2581,25 @@ fn resolve_token_budget_config(
         ));
     }
 
+    let guidance_message = token_budget_config
+        .and_then(|config| config.guidance_message.clone())
+        .filter(|message| !message.trim().is_empty());
+    if guidance_message
+        .as_ref()
+        .is_some_and(|message| message.len() > TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES)
+    {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!(
+                "features.token_budget.guidance_message must not exceed {TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES} bytes"
+            ),
+        ));
+    }
+
     Ok(Some(TokenBudgetConfig {
         reminder_threshold_tokens,
         reminder_message_template,
+        guidance_message,
     }))
 }