feat: add PackageUpgrade to Capabilities, exhaustive detect + CLI tests

- Add PackageUpgrade field to Capabilities struct and GetCapabilities
- Add PackageUpgrade to all 11 provider capability tests
- Add pkg-upgrade to CLI detect command output
- Expand detect tests: ByName for all managers, concurrent Reset,
  HasBinary, candidates/allManagers coverage
- Add cmd/snack unit tests: targets, opts, getManager, version
- 838 tests passing, 0 failures

dpkg/dpkg_test.go

@@ -117,6 +117,7 @@ func TestCapabilities(t *testing.T) {
 		{"RepoManagement", caps.RepoManagement, false},
 		{"KeyManagement", caps.KeyManagement, false},
 		{"Groups", caps.Groups, false},
+		{"PackageUpgrade", caps.PackageUpgrade, false},
 	}
 	for _, c := range checks {
 		t.Run(c.name, func(t *testing.T) {