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-0002-clean-break-from-v0.1.0.md

@@ -0,0 +1,24 @@
+# ASR-0002: Clean break from go-env v0.1.0
+
+## Context
+
+`go-env` v0.1.0 exposed package-level helpers (`Unmarshal`, `UnmarshalPrefix`, `AsMap`, …). The new design is `Codec`-centric with multiple sources per format.
+
+## Decision
+
+1. **No compatibility shims** in `go-config` re-exporting the old API.
+2. v0.1.0 remains tagged on git history as `github.com/eslider/go-env@v0.1.0` for archival consumers.
+3. Migration path is documented in `CHANGELOG.md` and the README.
+
+## Consequences
+
+- Single-call-site migrations must switch to `env.New(env.WithCurrentEnvironment(), …).Unmarshal(&cfg)`.
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- CHANGELOG `[0.2.0]`
+- ASR-0001