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

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

codex-rs/core/src/state/service.rs

@@ -44,7 +44,7 @@ pub(crate) struct SessionServices {
     pub(crate) user_shell: Arc<crate::shell::Shell>,
     pub(crate) shell_snapshot_tx: watch::Sender<Option<Arc<crate::shell_snapshot::ShellSnapshot>>>,
     pub(crate) show_raw_agent_reasoning: bool,
-    pub(crate) exec_policy: ExecPolicyManager,
+    pub(crate) exec_policy: Arc<ExecPolicyManager>,
     pub(crate) auth_manager: Arc<AuthManager>,
     pub(crate) models_manager: Arc<ModelsManager>,
     pub(crate) session_telemetry: SessionTelemetry,