feat(apt,dpkg): implement apt and dpkg package manager wrappers

Implement the snack.Manager interface for both apt (Debian/Ubuntu) and
dpkg (low-level Debian package tool).

apt wraps apt-get, apt-cache, and dpkg-query for full package management.
dpkg wraps dpkg and dpkg-query for low-level .deb operations.
Upgrade and Update return ErrUnsupportedPlatform for dpkg.

Both packages include:
- Linux implementations with proper error wrapping
- Non-linux build stubs returning ErrUnsupportedPlatform
- Output parsing helpers with tests
- Functional options support (sudo, assume-yes, dry-run)

apt/parse.go

@@ -0,0 +1,77 @@
+package apt
+
+import (
+	"strings"
+
+	"github.com/gogrlx/snack"
+)
+
+// parseList parses dpkg-query -W output into packages.
+func parseList(output string) []snack.Package {
+	var pkgs []snack.Package
+	for _, line := range strings.Split(strings.TrimSpace(output), "\n") {
+		if line == "" {
+			continue
+		}
+		parts := strings.SplitN(line, "\t", 3)
+		if len(parts) < 2 {
+			continue
+		}
+		p := snack.Package{
+			Name:      parts[0],
+			Version:   parts[1],
+			Installed: true,
+		}
+		if len(parts) == 3 {
+			p.Description = parts[2]
+		}
+		pkgs = append(pkgs, p)
+	}
+	return pkgs
+}
+
+// parseSearch parses apt-cache search output.
+func parseSearch(output string) []snack.Package {
+	var pkgs []snack.Package
+	for _, line := range strings.Split(strings.TrimSpace(output), "\n") {
+		if line == "" {
+			continue
+		}
+		// Format: "package - description"
+		parts := strings.SplitN(line, " - ", 2)
+		if len(parts) < 1 {
+			continue
+		}
+		p := snack.Package{Name: strings.TrimSpace(parts[0])}
+		if len(parts) == 2 {
+			p.Description = strings.TrimSpace(parts[1])
+		}
+		pkgs = append(pkgs, p)
+	}
+	return pkgs
+}
+
+// parseInfo parses apt-cache show output into a Package.
+func parseInfo(output string) (*snack.Package, error) {
+	p := &snack.Package{}
+	for _, line := range strings.Split(output, "\n") {
+		key, val, ok := strings.Cut(line, ": ")
+		if !ok {
+			continue
+		}
+		switch key {
+		case "Package":
+			p.Name = val
+		case "Version":
+			p.Version = val
+		case "Description":
+			p.Description = val
+		case "Architecture":
+			p.Arch = val
+		}
+	}
+	if p.Name == "" {
+		return nil, snack.ErrNotFound
+	}
+	return p, nil
+}