feat(list_dir): lazy BFS expansion and cache-dir collapsing

Expand lazily instead of walking the seed eagerly, and list agent/cache
dirs (.graff, .harness, .claude, .codex, .engram, node_modules,
zig-out, .zig-cache, venvs, tool caches) as collapsed entries without
descending unless they are the requested path. Dotfiles sort after
non-dot dirs, files follow dirs. Keeps the 10k budget for source trees
instead of spending it inside scratch and package caches.