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
-}