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/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())