## Bug

Running `omarchy install terminal ghostty` on aarch64 prints "Failed to install ghostty" but still:

1. Copies `/usr/share/omarchy/config/ghostty/` into `~/.config/ghostty/`
2. Overwrites `~/.config/xdg-terminals.list` so it points at `com.mitchellh.ghostty.desktop`

The package was never installed (`pacman -Q | grep ghostty` is empty afterwards, no `ghostty` binary on PATH), but `xdg-terminal-exec` and any Hyprland keybinding that resolves the default terminal now look for ghostty and fail to launch a terminal.

## Steps to reproduce

1. Use Omarchy on aarch64 (Apple Silicon). Ghostty is not in the aarch64 repos and there is no `ghostty` package in AUR (`aur/ghostty` does not exist; only derivatives like `ghostty-git` / `ghostty-nightly-bin`).
2. Run `omarchy install terminal ghostty`.

## What happens

```
Installing ghostty...
Skipping 'ghostty': not available in the repos on this system
```

But then the script silently:

- creates `~/.config/ghostty/` (copied from `/usr/share/omarchy/config/ghostty/`)
- rewrites `~/.config/xdg-terminals.list` to:
  ```
  com.mitchellh.ghostty.desktop
  ```

The user has no terminal they can launch and no package they can run.

## Expected

When the package cannot be installed, `omarchy install terminal` should:
- Print the error and exit non-zero
- Leave `~/.config/xdg-terminals.list` unchanged
- Leave `~/.config/<package>/` untouched

## Root cause

`omarchy install terminal` (in `/usr/bin/omarchy-install-terminal`) gates the config-copy and `xdg-terminals.list` rewrite on `if omarchy-pkg-add $package; then`. But `omarchy-pkg-add` (in `/usr/bin/omarchy-pkg-add`) does `exit 0` when none of the requested packages are installable:

```bash
to_install=()
for pkg in "$@"; do
  if pacman -Q "$pkg" &>/dev/null || pacman -Si "$pkg" &>/dev/null; then
    to_install+=("$pkg")
  else
    echo -e "\033[33mSkipping '$pkg': not available in the repos on this system\033[0m" >&2
  fi
done

((${#to_install[@]})) || exit 0
```

So the `if` branch fires, even though no package was actually installed. The script needs a separate "did anything actually get installed?" check (e.g. `omarchy-pkg-present $package` after the call, or a non-zero exit from `omarchy-pkg-add` when nothing was installable).

## Environment

- Omarchy 4.0.1-2
- aarch64 (Apple M1, Icestorm)
- Kernel: Linux 7.1.6-1-1-ARCH