[codex] Load user instructions through an injected provider (#27101)

## Why

We want to remove implicit use of `$CODEX_HOME` from `codex-core` and
make embedders responsible for supplying user-level instructions. This
also ensures user instructions load when no primary environment is
selected.

## What changed

Stacked on #27415, which makes `codex exec` surface thread-scoped
runtime warnings.

- Added `UserInstructionsProvider` to `codex-extension-api`, with
absolute source attribution and recoverable loading warnings.
- Added `codex-home` with the filesystem-backed provider for
`AGENTS.override.md` and `AGENTS.md`, preserving precedence, fallback,
trimming, lossy UTF-8 handling, and the existing uncapped global
instruction size.
- Removed global instruction loading from `Config` and require
`ThreadManager` callers to inject a provider.
- Load provider instructions once for each fresh root runtime, including
runtimes without a primary environment. Running sessions retain their
snapshot, while child agents inherit the parent snapshot without
invoking the provider.
- Keep provider instructions separate while loading project `AGENTS.md`,
then assemble the model-visible instructions with the existing ordering,
source attribution, warning, and turn-context behavior.
- Wired the Codex home provider through the CLI, app server, MCP server,
core facade, and thread-manager sample.

## Validation

- `just test -p codex-home -p codex-extension-api`
- `just test -p codex-core agents_md`
- `just test -p codex-core guardian`
- `just test -p codex-app-server
thread_start_without_selected_environment_includes_only_global_instruction_source`
- `just test -p codex-exec warning`
- `just bazel-lock-check`

codex-rs/exec/tests/suite/agents_md.rs

@@ -142,3 +142,35 @@ async fn exec_json_surfaces_project_instruction_loading_warnings() -> anyhow::Re
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn exec_surfaces_global_instruction_loading_warnings() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+    let global_agents_path = test.home_path().join("AGENTS.md");
+    let global_agents_source_suffix = format!(
+        "{}{}AGENTS.md",
+        test.home_path()
+            .file_name()
+            .expect("temporary Codex home should have a file name")
+            .to_string_lossy(),
+        std::path::MAIN_SEPARATOR,
+    );
+    std::fs::write(&global_agents_path, b"global\xFFinstructions")?;
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp1"),
+        responses::ev_assistant_message("m1", "fixture hello"),
+        responses::ev_completed("resp1"),
+    ]);
+    responses::mount_sse_once(&server, body).await;
+
+    test.cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("tell me something")
+        .assert()
+        .success()
+        .stderr(contains("invalid UTF-8").and(contains(global_agents_source_suffix)));
+
+    Ok(())
+}