Make AGENTS.md discovery FS-aware (#15826)

## Summary
- make AGENTS.md discovery and loading fully FS-aware and remove the
non-FS discover helper
- migrate remote-aware codex-core tests to use TestEnv workspace setup
instead of syncing a local workspace copy
- add AGENTS.md corner-case coverage, including directory fallbacks and
remote-aware integration coverage

## Testing
- cargo test -p codex-core project_doc -- --nocapture
- cargo test -p codex-core hierarchical_agents -- --nocapture
- cargo test -p codex-core agents_md -- --nocapture
- cargo test -p codex-tui status -- --nocapture
- cargo test -p codex-tui-app-server status -- --nocapture
- just fix
- just fmt
- just bazel-lock-update
- just bazel-lock-check
- just argument-comment-lint
- remote Linux executor tests in progress via scripts/test-remote-env.sh

codex-rs/core/tests/suite/remote_env.rs

@@ -1,6 +1,6 @@
 use anyhow::Result;
 use codex_exec_server::RemoveOptions;
-use codex_utils_absolute_path::AbsolutePathBuf;
+use core_test_support::PathBufExt;
 use core_test_support::get_remote_test_env;
 use core_test_support::test_codex::test_env;
 use pretty_assertions::assert_eq;
@@ -17,8 +17,7 @@ async fn remote_test_env_can_connect_and_use_filesystem() -> Result<()> {
     let test_env = test_env().await?;
     let file_system = test_env.environment().get_filesystem();
 
-    let file_path = remote_test_file_path();
-    let file_path_abs = absolute_path(file_path.clone())?;
+    let file_path_abs = remote_test_file_path().abs();
     let payload = b"remote-test-env-ok".to_vec();
 
     file_system
@@ -39,12 +38,6 @@ async fn remote_test_env_can_connect_and_use_filesystem() -> Result<()> {
 
     Ok(())
 }
-
-fn absolute_path(path: PathBuf) -> Result<AbsolutePathBuf> {
-    AbsolutePathBuf::try_from(path.clone())
-        .map_err(|err| anyhow::anyhow!("invalid absolute path {}: {err}", path.display()))
-}
-
 fn remote_test_file_path() -> PathBuf {
     let nanos = match SystemTime::now().duration_since(UNIX_EPOCH) {
         Ok(duration) => duration.as_nanos(),