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/merge/expected-replace.yaml

@@ -0,0 +1,12 @@
+service:
+  name: default-name
+  sub-service:
+    name: overlay-name
+    key: default-key
+    timeout: 1
+    enabled: false
+  hosts:
+    - x
+  database:
+    url: postgres://old
+    pool-size: 1