Adds `cmux tree` that prints the window > workspace > pane > surface
hierarchy with box-drawing characters. Includes server-side system.tree
RPC for single-round-trip performance.
Features:
- --all flag for all windows (default: current window only)
- --workspace flag to filter to a single workspace
- --json for structured JSON output
- Active path markers (◀ active) and caller identification (◀ here)
- Browser surfaces show their current URL
Closes https://github.com/manaflow-ai/cmux/issues/586
