fs: support pruning hidden directories during walks (#31570)

Why

Filtering hidden directories after a walk is too late: their descendants
consume traversal limits, and canonical directory deduplication can let
a hidden path claim a target before a visible symlink reaches it.

What this changes

- Add an optional pruneHiddenDirectories walk option. It defaults to
false and is omitted from the wire when disabled.
- Return hidden directory entries, but do not traverse them or add their
canonical identities to the visited set.
- Cover the visible-symlink-to-hidden-directory case through both local
and remote filesystem implementations.

This is the small filesystem prerequisite for #31566. Skill-specific
behavior remains in that PR.

codex-rs/core-skills/src/loader/environment.rs

@@ -160,6 +160,7 @@ pub async fn load_environment_skills_from_root(
                 max_directories: MAX_SKILLS_DIRS_PER_ROOT,
                 max_entries: MAX_SKILLS_ENTRIES_PER_ROOT,
                 follow_directory_symlinks: true,
+                prune_hidden_directories: false,
             },
             /*sandbox*/ None,
         )