Medium: Add --exclude flag to skip specific package managers

[jstreitb]

Problem: Sometimes users want to run baa but skip a specific manager (e.g., "Update everything except Flatpak because it's too slow right now").

Solution: Add a --exclude (shorthand -e) flag that takes a comma-separated list of managers to skip.

Example:
baa --exclude flatpak,snap

Tasks:

• Handle the flag parsing in main.go.
• Filter the detected providers list before the update loop starts.
• Show a small note in the UI: "Skipping: flatpak, snap".