Add `rb status` command

[tale]

Add a rb status subcommand that shows the current state of all managed files:

• Which files are managed (written or symlinked by rootbeer)
• Whether they are up-to-date, modified externally, or missing
• Which symlinks are correct vs stale/broken

This requires some form of state tracking — at minimum, a manifest of managed paths written after each successful apply (e.g. ~/.local/share/rootbeer/state.json). This manifest can also power future cleanup of removed files (#TBD).