feat(platform): add PackageManager interface and Pacman implementation

internal/platform/pkgmgr.go

@@ -0,0 +1,19 @@
+package platform
+
+import "context"
+
+// PackageManager abstracts OS-level package operations so later plans
+// can add Debian/Fedora without touching modules.
+type PackageManager interface {
+	Name() string
+	IsInstalled(ctx context.Context, pkg string) (bool, error)
+	Install(ctx context.Context, pkgs []string) error
+	Update(ctx context.Context) error
+}
+
+// Runner executes a command with arguments. Injected into package managers
+// so tests can stub shell-outs.
+type Runner interface {
+	Run(ctx context.Context, name string, args ...string) error
+	Output(ctx context.Context, name string, args ...string) ([]byte, error)
+}