feat: add TOML codec and envc format support

New toml package (pelletier/go-toml/v2) matching yaml/json options and merge flow.
envc convert/get/merge accept --from/--to toml; fixture and CLI tests; docs refresh.

Co-authored-by: Cursor <cursoragent@cursor.com>

toml/codec_test.go

@@ -0,0 +1,41 @@
+package toml
+
+import (
+	"context"
+	"testing"
+
+	"github.com/eslider/go-config/internal/testfixtures"
+)
+
+func TestCodec_IdentityTOMLFixture(t *testing.T) {
+	c := New(WithBytes(testfixtures.Load(t, "identity", "service.toml")))
+	m, err := c.Map(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m["service"].(map[string]any)["name"] != "my-service" {
+		t.Fatalf("%#v", m)
+	}
+}
+
+func TestCodec_MarshalRoundTripMap(t *testing.T) {
+	in := map[string]any{
+		"app": map[string]any{
+			"name": "demo",
+			"port": int64(8080),
+		},
+	}
+	c := New()
+	b, err := c.Marshal(in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c2 := New(WithBytes(b))
+	m, err := c2.Map(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if m["app"].(map[string]any)["name"] != "demo" {
+		t.Fatalf("%#v", m)
+	}
+}