Read the architecture at run time instead of a Buildarch ldflag (closes #259)

The Makefile no longer passes the architecture to the linker. The
Buildarch variable in main and the field and setter in globals are
gone; the startup log line reports runtime.GOARCH as `arch`.
CONVENTIONS.md drops Buildarch from its main, globals, logger and
Makefile examples, as the sneak/prompts conventions do.

Model: opus-5-5

internal/logger/logger_test.go

@@ -0,0 +1,34 @@
+package logger //nolint:testpackage // sets the unexported log and params fields
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/upaas/internal/globals"
+)
+
+func TestIdentifyLogsArchitectureFromRuntime(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	l := &Logger{
+		log: slog.New(slog.NewJSONHandler(&buf, nil)),
+		params: Params{
+			Globals: &globals.Globals{Appname: "upaas-test", Version: "test"},
+		},
+	}
+
+	l.Identify()
+
+	var line map[string]any
+
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &line))
+	assert.Equal(t, runtime.GOARCH, line["arch"])
+}