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_detect_pm.fish

@@ -0,0 +1,13 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# Detect the first available system package manager.
+function _fish_deps_detect_pm
+    for pm in paru yay pacman apt brew pkg dnf yum
+        if type -q $pm
+            echo $pm
+            return
+        end
+    end
+    echo ""
+end