fs: support pruning hidden directories during walks

[jif-oai]

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.