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-0007-cli-envc-convert-get-merge.md

@@ -0,0 +1,24 @@
+# ASR-0007: CLI `envc` — convert, get, merge
+
+## Context
+
+Operators need a small tool to translate configuration between formats and to inspect merged trees without writing Go.
+
+## Decision
+
+1. Binary name **`envc`**; module path **`github.com/eslider/go-config/cmd/envc`** (`go install …/cmd/envc@latest`).
+2. Subcommands: **`convert`**, **`get`**, **`merge`** — each implemented as a **`Run*(args, stdin, stdout, stderr) int`** function for in-process tests.
+3. **No** third-party CLI framework in v1 of the CLI.
+
+## Consequences
+
+- `cmd/envc` may grow flags; keep business logic in subpackages, not in `main` beyond wiring.
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- inventar ASR-0008 §2 — CLI placement
+- ASR-0003