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/mod.rs

@@ -74,6 +74,7 @@ pub static CODEX_ALIASES_TEMP_DIR: Option<TestCodexAliasesGuard> = {
 mod abort_tasks;
 mod agent_jobs;
 mod agent_websocket;
+mod agents_md;
 mod apply_patch_cli;
 #[cfg(not(target_os = "windows"))]
 mod approvals;