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

snap/snap_test.go

@@ -444,6 +444,9 @@ func TestCapabilities(t *testing.T) {
 	if caps.NameNormalize {
 		t.Error("expected NameNormalize=false")
 	}
+	if !caps.PackageUpgrade {
+		t.Error("expected PackageUpgrade=true")
+	}
 }
 
 func TestName(t *testing.T) {