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>

internal/source/source_test.go

@@ -0,0 +1,43 @@
+package source
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestURL_OK(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte("hello"))
+	}))
+	defer srv.Close()
+
+	u := URL{Raw: srv.URL}
+	rc, err := u.Open(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = rc.Close() }()
+	b, err := io.ReadAll(rc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(b) != "hello" {
+		t.Fatalf("got %q", b)
+	}
+}
+
+func TestURL_StatusError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer srv.Close()
+
+	u := URL{Raw: srv.URL}
+	_, err := u.Open(context.Background())
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}