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>

env/map_flat.go

@@ -0,0 +1,49 @@
+package env
+
+import (
+	"strings"
+)
+
+// insertPath walks path inside root, creating intermediate nested maps as
+// needed, and stores value at the leaf. First write wins for a given leaf key
+// within a single source map.
+func insertPath(root map[string]any, path []string, value string) {
+	current := root
+	last := len(path) - 1
+	for i, raw := range path {
+		k := strings.ToLower(raw)
+
+		if i == last {
+			if _, exists := current[k]; exists {
+				return
+			}
+			current[k] = value
+			return
+		}
+
+		next, ok := current[k].(map[string]any)
+		if !ok {
+			next = make(map[string]any)
+			current[k] = next
+		}
+		current = next
+	}
+}
+
+func nestedFromFlat(flat map[string]string, prefix string) map[string]any {
+	out := make(map[string]any)
+	for k, v := range flat {
+		name := k
+		if prefix != "" {
+			if !strings.HasPrefix(name, prefix) {
+				continue
+			}
+			name = name[len(prefix):]
+			if name == "" {
+				continue
+			}
+		}
+		insertPath(out, strings.Split(name, "_"), v)
+	}
+	return out
+}