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>

docs/asr/ASR-0004-configurable-key-normalization.md

@@ -0,0 +1,23 @@
+# ASR-0004: Configurable key normalization
+
+## Context
+
+Real-world configuration mixes `kebab-case`, `snake_case`, `SCREAMING_SNAKE`, and struct field names. Requiring `mapstructure` tags on every field is noisy.
+
+## Decision
+
+1. After parsing and merging, codecs run **`internal/keymap.Walk`** with a **`Normalizer`** (default **`LowerAlnum`**: lowercase + strip non `[a-z0-9]`).
+2. Callers may set **`WithKeyNormalizer(nil)`** to disable walking, or supply a custom function.
+
+## Consequences
+
+- POSIX env keys without `-` still align with YAML keys that contain hyphens once normalized.
+- CLI `get` applies the same normalizer to each path segment for consistency.
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- ASR-0003