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-0006-third-party-library-choices.md

@@ -0,0 +1,26 @@
+# ASR-0006: Third-party library choices
+
+## Context
+
+We need battle-tested parsers and struct bridging without maintaining our own grammars.
+
+## Decision
+
+1. **Struct / map bridging:** `github.com/go-viper/mapstructure/v2` (maintained fork of `mitchellh/mapstructure`).
+2. **YAML:** `gopkg.in/yaml.v3`.
+3. **JSON:** `encoding/json` (stdlib).
+4. **INI:** `gopkg.in/ini.v1` with `Loose` parsing for tolerant files.
+5. **`.env` files:** `github.com/joho/godotenv` (`UnmarshalBytes`).
+6. **CLI:** stdlib `flag` only (no Cobra).
+
+## Consequences
+
+- `go.mod` carries the above direct dependencies (plus transitive modules as resolved by MVS).
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- [go-viper/mapstructure](https://github.com/go-viper/mapstructure)