fix(deps): fix autoloading and Fish 4.x test compatibility

Split _fish_deps_pm.fish into one file per function so Fish can
autoload each by name (_fish_deps_detect_pm, _fish_deps_pm_install,
_fish_deps_pm_upgrade). A single file with multiple functions only
autoloads under the filename, leaving the others unreachable.

Replace all `test -a`/`test -o` binary operators with `; and`/`; or`
chains throughout _fish_deps_install and _fish_deps_update. Fish 4.x
removed support for -a/-o in test, causing the "unexpected argument"
errors seen on Debian.

Also consolidate paru/yay cases in pm_install and pm_upgrade since
both helpers use identical flags.

functions/_fish_deps_pm_install.fish

@@ -0,0 +1,28 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# Install a package via the system PM.
+# Usage: _fish_deps_pm_install <pkg>
+function _fish_deps_pm_install --argument-names pkg
+    set -l pm (_fish_deps_detect_pm)
+    if test -z "$pm"
+        echo "No supported package manager found." >&2
+        return 1
+    end
+    switch $pm
+        case paru yay
+            $pm -S --noconfirm $pkg
+        case pacman
+            sudo pacman -S --noconfirm $pkg
+        case apt
+            sudo apt install -y $pkg
+        case brew
+            brew install $pkg
+        case pkg
+            sudo pkg install -y $pkg
+        case dnf
+            sudo dnf install -y $pkg
+        case yum
+            sudo yum install -y $pkg
+    end
+end