feat(go-config): multi-format codecs, envc CLI, fixtures, ASRs

- Module github.com/eslider/go-config with env/yaml/json/ini packages
- internal: source, keymap, merge, structconv, bytesutil
- cmd/envc: convert, get, merge
- docs/asr ASR-0001..0008, README/CHANGELOG/AGENTS refresh
- golangci-lint v2 config; tests + fixtures

Co-authored-by: Cursor <cursoragent@cursor.com>

yaml/error_test.go

@@ -0,0 +1,16 @@
+package yaml
+
+import (
+	"context"
+	"testing"
+
+	"github.com/eslider/go-config/testfixtures"
+)
+
+func TestCodec_InvalidYAML(t *testing.T) {
+	c := New(WithBytes(testfixtures.Load(t, "invalid", "malformed.yaml")))
+	_, err := c.Map(context.Background())
+	if err == nil {
+		t.Fatal("expected parse error")
+	}
+}