Follow directory symlinks in filesystem walks (#29844)

Stack 3 of 3. Stacked on #29842.

## What changes

Adds an opt-in `followDirectorySymlinks` setting to `fs/walk`.

When enabled, the walk follows directory symlinks but continues to
ignore symlinked files. Canonical directory identities prevent symlink
cycles, while normal paths keep their existing spelling.

Environment skill discovery enables the setting so symlinked skill
directories continue to work with the new single-RPC scan.

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

@@ -116,6 +116,7 @@ pub async fn load_environment_skills_from_root(
                 max_depth: MAX_SCAN_DEPTH,
                 max_directories: MAX_SKILLS_DIRS_PER_ROOT,
                 max_entries: MAX_SKILLS_ENTRIES_PER_ROOT,
+                follow_directory_symlinks: true,
             },
             /*sandbox*/ None,
         )