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/edge/deeply-nested.yaml

@@ -0,0 +1,5 @@
+a:
+  b:
+    c:
+      d:
+        e: leaf

fixtures/edge/key-styles.yaml

@@ -0,0 +1,4 @@
+topLevel: 1
+sub-Service: 2
+snake_case: 3
+CamelCase: 4

fixtures/edge/types.json

@@ -0,0 +1,7 @@
+{
+  "flag": true,
+  "count": 42,
+  "ratio": 3.14,
+  "nothing": null,
+  "nested": [1, 2]
+}