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>

fixtures/identity/service.json

@@ -0,0 +1,15 @@
+{
+  "service": {
+    "name": "my-service",
+    "sub-service": {
+      "name": "abc",
+      "key": "abs",
+      "timeout": 30,
+      "enabled": true
+    },
+    "database": {
+      "url": "postgres://localhost:5432/db",
+      "pool-size": 10
+    }
+  }
+}