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/session/multi_agents.rs

@@ -78,7 +78,11 @@ pub(super) fn usage_hint_text(
         .as_ref()
         .and_then(|messages| messages.multi_agent.as_ref())
         .and_then(|messages| messages.role.as_ref());
-    let snapshot = resolve_usage_hints(&turn_context.config.multi_agent_v2, catalog);
+    let snapshot = resolve_usage_hints(
+        &turn_context.config.multi_agent_v2,
+        catalog,
+        !turn_context.config.update_plan_enabled && turn_context.config.model_catalog.is_none(),
+    );
     match session_source {
         SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. }) => snapshot.subagent,
         SessionSource::Cli
@@ -94,6 +98,7 @@ pub(super) fn usage_hint_text(
 pub(crate) fn resolve_usage_hints(
     config: &MultiAgentV2Config,
     catalog: Option<&MultiAgentRoleMessages>,
+    omit_update_plan_instructions: bool,
 ) -> ResolvedMultiAgentV2UsageHints {
     let resolve_role = |configured: Option<&str>, catalog: Option<&str>, bundled: &str| {
         // Configured roles take precedence; empty configured or catalog roles suppress fallback.
@@ -106,6 +111,11 @@ pub(crate) fn resolve_usage_hints(
         if base.is_empty() {
             return None;
         }
+        let base = if omit_update_plan_instructions {
+            crate::context::without_update_plan_instructions(base)
+        } else {
+            base.to_string()
+        };
 
         let max_concurrency = config.max_concurrent_threads_per_session;
         let wait_agent_guidance = if config.wait_agent_enabled {