feat(aur): implement native AUR client

Native Go implementation using the AUR RPC v5 API for search/info
queries and git+makepkg+pacman for building and installing packages.

Implements:
- Manager (full: Install, Remove, Purge, Upgrade, Update, List, Search,
  Info, IsInstalled, Version)
- VersionQuerier (LatestVersion, ListUpgrades, UpgradeAvailable, VersionCmp)
- Cleaner (Autoremove via pacman, Clean build dir)
- PackageUpgrader (UpgradePackages)

Key design decisions:
- No CLI wrapper (paru/yay) — uses AUR RPC API directly + git clone
- Packages are built with makepkg and installed via pacman -U
- Foreign packages (pacman -Qm) are treated as AUR packages for List
- Batch RPC queries (rpcInfoMulti) for efficient upgrade checks
- Configurable build directory and makepkg flags
- Not added to detect.Default() candidates (AUR supplements pacman,
  not replaces it) but available via detect.ByName("aur")

detect/detect_linux.go

@@ -6,6 +6,7 @@ import (
 	"github.com/gogrlx/snack"
 	"github.com/gogrlx/snack/apk"
 	"github.com/gogrlx/snack/apt"
+	"github.com/gogrlx/snack/aur"
 	"github.com/gogrlx/snack/dnf"
 	"github.com/gogrlx/snack/flatpak"
 	"github.com/gogrlx/snack/pacman"
@@ -26,6 +27,7 @@ func candidates() []managerFactory {
 }
 
 // allManagers returns all known manager factories (for ByName).
+// Includes supplemental managers like AUR that aren't primary candidates.
 func allManagers() []managerFactory {
-	return candidates()
+	return append(candidates(), func() snack.Manager { return aur.New() })
 }