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-0005-multi-source-deep-merge-semantics.md

@@ -0,0 +1,26 @@
+# ASR-0005: Multi-source deep-merge semantics
+
+## Context
+
+Layered configuration (defaults file < local overrides < process environment) requires predictable composition rules.
+
+## Decision
+
+1. Sources are applied in **option order** (left → right). Each source parses to a `map[string]any`, then **`merge.DeepMerge`** folds them.
+2. **Maps recurse** — nested keys are unioned; existing nested maps are not replaced wholesale by a sibling scalar.
+3. **Scalar leaves** use **last-write-wins** (later source wins).
+4. **Slices** default to **`merge.Replace`**; **`WithSliceMerge(merge.Concat)`** appends prior and new slice elements.
+
+## Consequences
+
+- Env files and `WithCurrentEnvironment()` compose when ordered **lowest → highest** priority.
+- Tests lock behaviour via `fixtures/merge/*`.
+
+## Status
+
+Accepted — 2026-05-02.
+
+## References
+
+- ASR-0003
+- `internal/merge`