feat: initial project scaffold

- Common Manager interface, Package type, functional options
- Sentinel errors for common package manager failures
- Sub-package stubs for: pacman, aur, apk, apt, dpkg, dnf, rpm,
  flatpak, snap, pkg (FreeBSD), ports (OpenBSD)
- detect/ package for auto-detection of system package manager
- 0BSD license

detect/detect.go

@@ -0,0 +1,43 @@
+// Package detect provides auto-detection of the system's available package manager.
+package detect
+
+import (
+	"os/exec"
+
+	"github.com/gogrlx/snack"
+)
+
+// probeOrder defines the order in which package managers are probed.
+// The first available manager wins.
+var probeOrder = []struct {
+	name string
+	bin  string
+}{
+	{"pacman", "pacman"},
+	{"apk", "apk"},
+	{"apt", "apt-get"},
+	{"dnf", "dnf"},
+	{"rpm", "rpm"},
+	{"flatpak", "flatpak"},
+	{"snap", "snap"},
+	{"pkg", "pkg"},
+}
+
+// Default returns the first available package manager on the system.
+// Returns ErrManagerNotFound if no supported manager is detected.
+func Default() (snack.Manager, error) {
+	// TODO: implement — probe for each manager in order, return first match
+	return nil, snack.ErrManagerNotFound
+}
+
+// All returns all available package managers on the system.
+func All() []snack.Manager {
+	// TODO: implement
+	return nil
+}
+
+// HasBinary reports whether a binary is available in PATH.
+func HasBinary(name string) bool {
+	_, err := exec.LookPath(name)
+	return err == nil
+}