Make the update_plan tool opt-in (#41744)

## What changed

- Default `tools.update_plan.enabled` to `false`; users can explicitly enable it to expose `update_plan`.
- Remove bundled `update_plan` guidance from model, collaboration-mode, multi-agent, compaction, prewarm, and goal-continuation prompts when the tool is disabled.
- Preserve custom base instructions, model catalog instructions, collaboration policies, and user goal text even when they mention planning or `update_plan`.

## Testing

- Cover default and explicitly enabled tool registration, prompt consistency across request paths, and preservation of custom instructions.

GitOrigin-RevId: a53964e6e72f98e2557dd3090fdb7caccb956527

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

@@ -566,8 +566,8 @@ async fn load_config_resolves_non_prefixed_mcp_tool_servers() -> std::io::Result
 async fn load_config_resolves_update_plan_enabled() -> std::io::Result<()> {
     let codex_home = tempdir()?;
     for (config_toml, expected_enabled) in [
-        ("", true),
-        ("[tools.update_plan]", true),
+        ("", false),
+        ("[tools.update_plan]", false),
         ("[tools.update_plan]\nenabled = false", false),
         ("[tools.update_plan]\nenabled = true", true),
     ] {
@@ -11604,7 +11604,9 @@ max_concurrent_threads_per_session = 17
     for wait_agent_enabled in [true, false] {
         let mut config = config.clone();
         config.wait_agent_enabled = wait_agent_enabled;
-        let usage_hints = resolve_usage_hints(&config, /*catalog*/ None);
+        let usage_hints = resolve_usage_hints(
+            &config, /*catalog*/ None, /*omit_update_plan_instructions*/ false,
+        );
         for hint in [usage_hints.root, usage_hints.subagent] {
             let hint = hint.expect("default usage hints should be present").body();
             assert!(hint.contains(concurrency_guidance));
@@ -11621,6 +11623,7 @@ max_concurrent_threads_per_session = 17
             root: Some(String::new()),
             subagent: Some(String::new()),
         }),
+        /*omit_update_plan_instructions*/ false,
     );
     assert!(usage_hints.root.is_none() && usage_hints.subagent.is_none());
 }
@@ -11628,12 +11631,12 @@ max_concurrent_threads_per_session = 17
 #[test]
 fn multi_agent_v2_model_override_exposure_preserves_configured_usage_hints() {
     let config_toml = toml::from_str(
-        r#"[features.multi_agent_v2]
+        r###"[features.multi_agent_v2]
 enabled = true
-root_agent_usage_hint_text = "Root guidance."
-subagent_usage_hint_text = "Subagent guidance."
+root_agent_usage_hint_text = "## Plan tool\nRoot guidance."
+subagent_usage_hint_text = "## `update_plan`\nSubagent guidance."
 expose_spawn_agent_model_overrides = true
-"#,
+"###,
     )
     .expect("multi-agent v2 config should parse");
 
@@ -11641,11 +11644,11 @@ expose_spawn_agent_model_overrides = true
     assert!(config.expose_spawn_agent_model_overrides);
     assert_eq!(
         config.root_agent_usage_hint_text.as_deref(),
-        Some("Root guidance.")
+        Some("## Plan tool\nRoot guidance.")
     );
     assert_eq!(
         config.subagent_usage_hint_text.as_deref(),
-        Some("Subagent guidance.")
+        Some("## `update_plan`\nSubagent guidance.")
     );
     let usage_hints = resolve_usage_hints(
         &config,
@@ -11653,6 +11656,7 @@ expose_spawn_agent_model_overrides = true
             root: Some("Catalog root base.".to_string()),
             subagent: Some("Catalog subagent base.".to_string()),
         }),
+        /*omit_update_plan_instructions*/ true,
     );
     assert_eq!(
         (
@@ -11660,8 +11664,8 @@ expose_spawn_agent_model_overrides = true
             usage_hints.subagent.map(|hint| hint.body()),
         ),
         (
-            Some("Root guidance.".to_string()),
-            Some("Subagent guidance.".to_string()),
+            Some("## Plan tool\nRoot guidance.".to_string()),
+            Some("## `update_plan`\nSubagent guidance.".to_string()),
         )
     );
 }
@@ -11673,9 +11677,13 @@ fn multi_agent_v2_exposes_model_overrides_by_default() {
 
     let mut config = resolve_multi_agent_v2_config(&config_toml);
     assert!(config.expose_spawn_agent_model_overrides);
-    let usage_hints = resolve_usage_hints(&config, /*catalog*/ None);
+    let usage_hints = resolve_usage_hints(
+        &config, /*catalog*/ None, /*omit_update_plan_instructions*/ false,
+    );
     config.expose_spawn_agent_model_overrides = false;
-    let usage_hints_without_model_overrides = resolve_usage_hints(&config, /*catalog*/ None);
+    let usage_hints_without_model_overrides = resolve_usage_hints(
+        &config, /*catalog*/ None, /*omit_update_plan_instructions*/ false,
+    );
 
     for (hint, hint_without_model_overrides) in [
         (usage_hints.root, usage_hints_without_model_overrides.root),
@@ -11790,6 +11798,7 @@ subagent_usage_hint_text = ""
             root: Some("catalog root".to_string()),
             subagent: Some("catalog subagent".to_string()),
         }),
+        /*omit_update_plan_instructions*/ false,
     );
     assert_eq!(
         (

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

@@ -2641,7 +2641,7 @@ fn resolve_update_plan_enabled(config_toml: &ConfigToml) -> bool {
         .tools
         .as_ref()
         .and_then(|tools| tools.update_plan.as_ref())
-        .is_none_or(|config| config.enabled)
+        .is_some_and(|config| config.enabled)
 }
 
 fn resolve_orchestrator_feature_enabled(