feat(home): Add starship cross-shell prompt role

Add a new home-manager role for starship.rs, a fast and customizable
cross-shell prompt written in Rust.

Configuration includes:
- Bash and Zsh integration enabled
- Clean character symbols (> for success, x for error)
- Vi mode indicator support
- Smart directory truncation (4 levels, truncate to repo root)
- Git branch and status display
- Nix shell indicator with snowflake symbol
- Command duration for long-running commands (2s+)
- Disabled noisy modules (language runtimes, cloud providers)

Enabled in: home-desktop, home-laptop-compact, home-live-usb,
home-media-center configurations.

Closes: nixos-configs-uji

home/roles/default.nix

@@ -19,5 +19,6 @@
     ./sync
     ./tmux
     ./emacs
+    ./starship
   ];
 }