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/globals/globals.go

@@ -12,17 +12,15 @@ import (
 //
 //nolint:gochecknoglobals // Required for ldflags injection at build time
 var (
-	mu        sync.RWMutex
-	appname   string
-	version   string
-	buildarch string
+	mu      sync.RWMutex
+	appname string
+	version string
 )
 
 // Globals holds build-time variables for dependency injection.
 type Globals struct {
-	Appname   string
-	Version   string
-	Buildarch string
+	Appname string
+	Version string
 }
 
 // New creates a new Globals instance from package-level variables.
@@ -31,9 +29,8 @@ func New(_ fx.Lifecycle) (*Globals, error) {
 	defer mu.RUnlock()
 
 	return &Globals{
-		Appname:   appname,
-		Version:   version,
-		Buildarch: buildarch,
+		Appname: appname,
+		Version: version,
 	}, nil
 }
 
@@ -52,11 +49,3 @@ func SetVersion(ver string) {
 
 	version = ver
 }
-
-// SetBuildarch sets the build architecture (used for testing and main init).
-func SetBuildarch(arch string) {
-	mu.Lock()
-	defer mu.Unlock()
-
-	buildarch = arch
-}

internal/logger/logger.go

@@ -4,6 +4,7 @@ package logger
 import (
 	"log/slog"
 	"os"
+	"runtime"
 
 	"go.uber.org/fx"
 
@@ -81,6 +82,6 @@ func (l *Logger) Identify() {
 	l.log.Info("starting",
 		"appname", l.params.Globals.Appname,
 		"version", l.params.Globals.Version,
-		"buildarch", l.params.Globals.Buildarch,
+		"arch", runtime.GOARCH,
 	)
 }

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"])
+}