feat: implement detect package and snack CLI

- detect: auto-detection with Default(), All(), ByName()
  - Platform-specific probing via build tags (linux, freebsd, openbsd)
  - Linux probe order: apt, dnf, pacman, apk, flatpak, snap
- cmd/snack: full CLI with cobra + charmbracelet/fang
  - Subcommands: install, remove, purge, upgrade, update, list, search,
    info, which, hold, unhold, clean, detect, version
  - Global flags: --manager, --sudo, --yes, --dry-run
  - Capability type-assertions for FileOwner, Holder, Cleaner
- detect: basic compilation tests

detect/detect_openbsd.go

@@ -0,0 +1,13 @@
+//go:build openbsd
+
+package detect
+
+// candidates returns manager factories in probe order for OpenBSD.
+// TODO: wire up ports.New() once the ports package is implemented.
+func candidates() []managerFactory {
+	return nil
+}
+
+func allManagers() []managerFactory {
+	return nil
+}