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/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,