feat: add Homebrew provider, implement NameNormalizer across all managers

- Add brew package for Homebrew support on macOS and Linux
- Implement NameNormalizer interface (NormalizeName, ParseArch) for all providers
- Add darwin platform detection with Homebrew as default
- Consolidate capabilities by removing separate *_linux.go/*_other.go files
- Update tests for new capability expectations
- Add comprehensive tests for AUR and brew providers
- Update README with capability matrix and modern Target API usage

💘 Generated with Crush

Assisted-by: AWS Claude Opus 4.5 via Crush <crush@charm.land>

ports/ports_other.go

@@ -49,3 +49,39 @@ func isInstalled(_ context.Context, _ string) (bool, error) {
 func version(_ context.Context, _ string) (string, error) {
 	return "", snack.ErrUnsupportedPlatform
 }
+
+func autoremove(_ context.Context, _ ...snack.Option) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func clean(_ context.Context) error {
+	return snack.ErrUnsupportedPlatform
+}
+
+func fileList(_ context.Context, _ string) ([]string, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func owner(_ context.Context, _ string) (string, error) {
+	return "", snack.ErrUnsupportedPlatform
+}
+
+func latestVersion(_ context.Context, _ string) (string, error) {
+	return "", snack.ErrUnsupportedPlatform
+}
+
+func listUpgrades(_ context.Context) ([]snack.Package, error) {
+	return nil, snack.ErrUnsupportedPlatform
+}
+
+func upgradeAvailable(_ context.Context, _ string) (bool, error) {
+	return false, snack.ErrUnsupportedPlatform
+}
+
+func versionCmp(_ context.Context, _, _ string) (int, error) {
+	return 0, snack.ErrUnsupportedPlatform
+}
+
+func upgradePackages(_ context.Context, _ []snack.Target, _ ...snack.Option) (snack.InstallResult, error) {
+	return snack.InstallResult{}, snack.ErrUnsupportedPlatform
+}