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>

go.mod

@@ -1,5 +1,12 @@
-module github.com/eslider/go-env
+module github.com/eslider/go-config
 
 go 1.22.2
 
-require github.com/mitchellh/mapstructure v1.5.0
+require (
+	github.com/go-viper/mapstructure/v2 v2.5.0
+	github.com/joho/godotenv v1.5.1
+	gopkg.in/ini.v1 v1.67.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require github.com/stretchr/testify v1.11.1 // indirect