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/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)
         }