codex: route global AGENTS reads through LOCAL_FS (#23343)

## Summary
- make `load_global_instructions` read through an `ExecutorFileSystem`
- call global AGENTS reads with explicit `LOCAL_FS` so they stay tied to
local codex-home state

## Validation
- `bazel test --bes_backend= --bes_results_url=
--test_filter=instruction_sources_include_global_before_agents_md_docs
//codex-rs/core:core-unit-tests` on `dev`

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

@@ -2417,8 +2417,10 @@ impl Config {
             guardian_policy_config_source: _,
         } = config_layer_stack.requirements().clone();
 
-        let user_instructions = AgentsMdManager::load_global_instructions(Some(&codex_home))
-            .map(|loaded| loaded.contents);
+        let user_instructions =
+            AgentsMdManager::load_global_instructions(LOCAL_FS.as_ref(), Some(&codex_home))
+                .await
+                .map(|loaded| loaded.contents);
         let mut startup_warnings = config_layer_stack
             .startup_warnings()
             .unwrap_or_default()