Warn on invalid UTF-8 in AGENTS.md files (#23232)

Fixes #23223.

## Why

Malformed AGENTS instructions should not fail silently. The reported
issue had invalid UTF-8 in a global `AGENTS.md`; before this change,
Codex treated that decode failure like a missing file, so the personal
instructions disappeared without a user-visible explanation and the
rollout had no `# AGENTS.md instructions` block.

Project-level AGENTS files already used lossy decoding, so their
instructions still appeared, but invalid bytes were replaced without
telling the user. Global and project AGENTS files should behave
consistently: keep usable instruction text when possible, and surface a
diagnostic when bytes had to be replaced.

## What changed

Global `AGENTS.override.md` and `AGENTS.md` loading now reads bytes and
decodes with replacement characters on invalid UTF-8, matching
project-level AGENTS behavior. Both global and project AGENTS loading
now emit a startup warning when invalid UTF-8 is found, and both keep
the instruction text with invalid byte sequences replaced.

Missing files, non-file candidates, empty files, and the existing
`AGENTS.override.md` before `AGENTS.md` precedence keep their current
behavior.

## How users see it

The warnings flow through the existing startup warning surface.
App-server clients receive config-time startup warnings as
`configWarning` notifications during initialization, and thread startup
emits startup warnings as thread-scoped `warning` notifications.

Global AGENTS invalid UTF-8 warnings can appear on both surfaces.
Project-level AGENTS invalid UTF-8 warnings are discovered while
building thread instructions, so they appear as thread-scoped `warning`
notifications. Clients that render warning notifications in the
conversation surface show the message as a visible diagnostic instead of
silently hiding or altering instructions.

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

@@ -2438,14 +2438,17 @@ impl Config {
             guardian_policy_config_source: _,
         } = config_layer_stack.requirements().clone();
 
-        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()
             .to_vec();
+        let user_instructions = AgentsMdManager::load_global_instructions(
+            LOCAL_FS.as_ref(),
+            Some(&codex_home),
+            &mut startup_warnings,
+        )
+        .await
+        .map(|loaded| loaded.contents);
 
         // Destructure ConfigOverrides fully to ensure all overrides are applied.
         let ConfigOverrides {