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>

cmd/envc/source.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"io"
+	"os"
+	"strings"
+
+	"github.com/eslider/go-config/internal/source"
+)
+
+func openSource(pathOrURL string, stdin io.Reader) source.Source {
+	if pathOrURL == "" || pathOrURL == "-" {
+		return source.Reader{R: stdin, Name: "stdin"}
+	}
+	if strings.HasPrefix(pathOrURL, "http://") || strings.HasPrefix(pathOrURL, "https://") {
+		return source.URL{Raw: pathOrURL}
+	}
+	return source.File{Path: pathOrURL}
+}
+
+type nopCloser struct{ io.Writer }
+
+func (nopCloser) Close() error { return nil }
+
+func openOutput(path string, stdout io.Writer) (io.WriteCloser, error) {
+	if path == "" || path == "-" {
+		if wc, ok := stdout.(io.WriteCloser); ok {
+			return wc, nil
+		}
+		return nopCloser{stdout}, nil
+	}
+	return os.Create(path)
+}