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/exec-server/src/lib.rs

@@ -41,6 +41,7 @@ pub use file_system::FileMetadata;
 pub use file_system::FileSystemResult;
 pub use file_system::ReadDirectoryEntry;
 pub use file_system::RemoveOptions;
+pub use local_file_system::LOCAL_FS;
 pub use process::ExecBackend;
 pub use process::ExecProcess;
 pub use process::StartedExecProcess;

codex-rs/exec-server/src/local_file_system.rs

@@ -3,6 +3,8 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use std::path::Component;
 use std::path::Path;
 use std::path::PathBuf;
+use std::sync::Arc;
+use std::sync::LazyLock;
 use std::time::SystemTime;
 use std::time::UNIX_EPOCH;
 use tokio::io;
@@ -17,6 +19,9 @@ use crate::RemoveOptions;
 
 const MAX_READ_FILE_BYTES: u64 = 512 * 1024 * 1024;
 
+pub static LOCAL_FS: LazyLock<Arc<dyn ExecutorFileSystem>> =
+    LazyLock::new(|| -> Arc<dyn ExecutorFileSystem> { Arc::new(LocalFileSystem) });
+
 #[derive(Clone, Default)]
 pub(crate) struct LocalFileSystem;
 

codex-rs/exec-server/src/remote_file_system.rs

@@ -23,6 +23,7 @@ use crate::ReadDirectoryEntry;
 use crate::RemoveOptions;
 
 const INVALID_REQUEST_ERROR_CODE: i64 = -32600;
+const NOT_FOUND_ERROR_CODE: i64 = -32004;
 
 #[derive(Clone)]
 pub(crate) struct RemoteFileSystem {
@@ -151,6 +152,9 @@ impl ExecutorFileSystem for RemoteFileSystem {
 
 fn map_remote_error(error: ExecServerError) -> io::Error {
     match error {
+        ExecServerError::Server { code, message } if code == NOT_FOUND_ERROR_CODE => {
+            io::Error::new(io::ErrorKind::NotFound, message)
+        }
         ExecServerError::Server { code, message } if code == INVALID_REQUEST_ERROR_CODE => {
             io::Error::new(io::ErrorKind::InvalidInput, message)
         }

codex-rs/exec-server/src/rpc.rs

@@ -356,6 +356,14 @@ pub(crate) fn invalid_params(message: String) -> JSONRPCErrorError {
     }
 }
 
+pub(crate) fn not_found(message: String) -> JSONRPCErrorError {
+    JSONRPCErrorError {
+        code: -32004,
+        data: None,
+        message,
+    }
+}
+
 pub(crate) fn internal_error(message: String) -> JSONRPCErrorError {
     JSONRPCErrorError {
         code: -32603,

codex-rs/exec-server/src/server/file_system_handler.rs

@@ -26,6 +26,7 @@ use crate::RemoveOptions;
 use crate::local_file_system::LocalFileSystem;
 use crate::rpc::internal_error;
 use crate::rpc::invalid_request;
+use crate::rpc::not_found;
 
 #[derive(Clone, Default)]
 pub(crate) struct FileSystemHandler {
@@ -153,7 +154,9 @@ impl FileSystemHandler {
 }
 
 fn map_fs_error(err: io::Error) -> JSONRPCErrorError {
-    if err.kind() == io::ErrorKind::InvalidInput {
+    if err.kind() == io::ErrorKind::NotFound {
+        not_found(err.to_string())
+    } else if err.kind() == io::ErrorKind::InvalidInput {
         invalid_request(err.to_string())
     } else {
         internal_error(err.to_string())