Reuse guardian session across approvals (#14668)

## Summary
- reuse a guardian subagent session across approvals so reviews keep a
stable prompt cache key and avoid one-shot startup overhead
- clear the guardian child history before each review so prior guardian
decisions do not leak into later approvals
- include the `smart_approvals` -> `guardian_approval` feature flag
rename in the same PR to minimize release latency on a very tight
timeline
- add regression coverage for prompt-cache-key reuse without
prior-review prompt bleed

## Request
- Bug/enhancement request: internal guardian prompt-cache and latency
improvement request

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/common/context_snapshot.rs

@@ -22,6 +22,7 @@ pub enum ContextSnapshotRenderMode {
 pub struct ContextSnapshotOptions {
     render_mode: ContextSnapshotRenderMode,
     strip_capability_instructions: bool,
+    strip_agents_md_user_context: bool,
 }
 
 impl Default for ContextSnapshotOptions {
@@ -29,6 +30,7 @@ impl Default for ContextSnapshotOptions {
         Self {
             render_mode: ContextSnapshotRenderMode::RedactedText,
             strip_capability_instructions: false,
+            strip_agents_md_user_context: false,
         }
     }
 }
@@ -43,6 +45,11 @@ impl ContextSnapshotOptions {
         self.strip_capability_instructions = true;
         self
     }
+
+    pub fn strip_agents_md_user_context(mut self) -> Self {
+        self.strip_agents_md_user_context = true;
+        self
+    }
 }
 
 pub fn format_request_input_snapshot(
@@ -88,6 +95,12 @@ pub fn format_response_items_snapshot(items: &[Value], options: &ContextSnapshot
                                         {
                                             return None;
                                         }
+                                        if options.strip_agents_md_user_context
+                                            && role == "user"
+                                            && text.starts_with("# AGENTS.md instructions for ")
+                                        {
+                                            return None;
+                                        }
                                         return Some(format_snapshot_text(text, options));
                                     }
                                     let Some(content_type) =
@@ -451,6 +464,33 @@ mod tests {
         assert_eq!(rendered, "00:message/developer:<PERMISSIONS_INSTRUCTIONS>");
     }
 
+    #[test]
+    fn strip_agents_md_user_context_omits_agents_fragment_from_user_messages() {
+        let items = vec![json!({
+            "type": "message",
+            "role": "user",
+            "content": [
+                {
+                    "type": "input_text",
+                    "text": "# AGENTS.md instructions for /tmp/example\n\n<INSTRUCTIONS>\n- test\n</INSTRUCTIONS>"
+                },
+                {
+                    "type": "input_text",
+                    "text": "<environment_context>\n  <cwd>/tmp/example</cwd>\n</environment_context>"
+                }
+            ]
+        })];
+
+        let rendered = format_response_items_snapshot(
+            &items,
+            &ContextSnapshotOptions::default()
+                .render_mode(ContextSnapshotRenderMode::RedactedText)
+                .strip_agents_md_user_context(),
+        );
+
+        assert_eq!(rendered, "00:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>");
+    }
+
     #[test]
     fn redacted_text_mode_normalizes_environment_context_with_subagents() {
         let items = vec![json!({