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

capabilities_test.go

@@ -77,6 +77,7 @@ func TestGetCapabilities_BaseManager(t *testing.T) {
 	assert.False(t, caps.Groups)
 	assert.False(t, caps.NameNormalize)
 	assert.False(t, caps.DryRun)
+	assert.False(t, caps.PackageUpgrade)
 }
 
 func TestGetCapabilities_FullManager(t *testing.T) {
@@ -90,4 +91,5 @@ func TestGetCapabilities_FullManager(t *testing.T) {
 	assert.True(t, caps.Groups)
 	assert.True(t, caps.NameNormalize)
 	assert.True(t, caps.DryRun)
+	assert.True(t, caps.PackageUpgrade)
 }