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-0003-uniform-codec-api-and-source-abstraction.md

@@ -0,0 +1,25 @@
+# ASR-0003: Uniform codec API and Source abstraction
+
+## Context
+
+Users switch between YAML, JSON, INI, and env-file formats; a different function surface per format would increase cognitive load and test matrices.
+
+## Decision
+
+1. Every format package exports **`New(opts...) *Codec`** with **`Map`**, **`Unmarshal`**, **`UnmarshalContext`**, **`Marshal`**, **`WriteTo`**.
+2. Bytes, readers, files, and URLs are modeled by **`internal/source.Source`** with **`Open(ctx) (io.ReadCloser, error)`**.
+3. HTTP(S) sources support **`WithHTTPHeader`** and **`WithHTTPClient`**.
+
+## Consequences
+
+- Cross-format examples in the README stay structurally identical.
+- URL behaviour is testable with `httptest` against our client only.
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- ASR-0005 — merge semantics across multiple sources
+- ASR-0008 — httptest policy alignment