fix(subagents) share execpolicy by default (#13702)

## Summary
If a subagent requests approval, and the user persists that approval to
the execpolicy, it should (by default) propagate. We'll need to rethink
this a bit in light of coming Permissions changes, though I think this
is closer to the end state that we'd want, which is that execpolicy
changes to one permissions profile should be synced across threads.

## Testing
- [x] Added integration test

---------

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

codex-rs/core/src/codex.rs

@@ -376,6 +376,7 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) persist_extended_history: bool,
     pub(crate) metrics_service_name: Option<String>,
     pub(crate) inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
+    pub(crate) inherited_exec_policy: Option<Arc<ExecPolicyManager>>,
     pub(crate) user_shell_override: Option<shell::Shell>,
     pub(crate) parent_trace: Option<W3cTraceContext>,
 }
@@ -429,6 +430,7 @@ impl Codex {
             metrics_service_name,
             inherited_shell_snapshot,
             user_shell_override,
+            inherited_exec_policy,
             parent_trace: _,
         } = args;
         let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
@@ -485,11 +487,15 @@ impl Codex {
             // Guardian review should rely on the built-in shell safety checks,
             // not on caller-provided exec-policy rules that could shape the
             // reviewer or silently auto-approve commands.
-            ExecPolicyManager::default()
+            Arc::new(ExecPolicyManager::default())
+        } else if let Some(exec_policy) = &inherited_exec_policy {
+            Arc::clone(exec_policy)
         } else {
-            ExecPolicyManager::load(&config.config_layer_stack)
-                .await
-                .map_err(|err| CodexErr::Fatal(format!("failed to load rules: {err}")))?
+            Arc::new(
+                ExecPolicyManager::load(&config.config_layer_stack)
+                    .await
+                    .map_err(|err| CodexErr::Fatal(format!("failed to load rules: {err}")))?,
+            )
         };
 
         let config = Arc::new(config);
@@ -1386,7 +1392,7 @@ impl Session {
         config: Arc<Config>,
         auth_manager: Arc<AuthManager>,
         models_manager: Arc<ModelsManager>,
-        exec_policy: ExecPolicyManager,
+        exec_policy: Arc<ExecPolicyManager>,
         tx_event: Sender<Event>,
         agent_status: watch::Sender<AgentStatus>,
         initial_history: InitialHistory,