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>

testfixtures/testfixtures.go

@@ -0,0 +1,46 @@
+// Package testfixtures resolves paths to files under the module's fixtures/ directory.
+package testfixtures
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func moduleRoot(t *testing.T) string {
+	t.Helper()
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		t.Fatal("runtime.Caller")
+	}
+	dir := filepath.Dir(file)
+	for {
+		data, err := os.ReadFile(filepath.Join(dir, "go.mod"))
+		if err == nil && strings.Contains(string(data), "module github.com/eslider/go-config") {
+			return dir
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			t.Fatal("module root not found")
+		}
+		dir = parent
+	}
+}
+
+// Path joins repo fixtures/ with parts.
+func Path(t *testing.T, parts ...string) string {
+	t.Helper()
+	return filepath.Join(append([]string{moduleRoot(t), "fixtures"}, parts...)...)
+}
+
+// Load reads a fixture file relative to fixtures/.
+func Load(t *testing.T, parts ...string) []byte {
+	t.Helper()
+	b, err := os.ReadFile(Path(t, parts...))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}