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>

flatpak/flatpak_test.go

@@ -397,8 +397,8 @@ func TestCapabilities(t *testing.T) {
 	if caps.Groups {
 		t.Error("expected Groups=false")
 	}
-	if caps.NameNormalize {
-		t.Error("expected NameNormalize=false")
+	if !caps.NameNormalize {
+		t.Error("expected NameNormalize=true")
 	}
 	if !caps.PackageUpgrade {
 		t.Error("expected PackageUpgrade=true")