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

cmd/upaasd/main.go

@@ -25,15 +25,13 @@ import (
 // Build-time variables injected by linker flags (-ldflags).
 // These must be exported package-level variables for the build system.
 var (
-	Appname   = "upaas" //nolint:gochecknoglobals // build-time variable
-	Version   string    //nolint:gochecknoglobals // build-time variable
-	Buildarch string    //nolint:gochecknoglobals // build-time variable
+	Appname = "upaas" //nolint:gochecknoglobals // build-time variable
+	Version string    //nolint:gochecknoglobals // build-time variable
 )
 
 func main() {
 	globals.SetAppname(Appname)
 	globals.SetVersion(Version)
-	globals.SetBuildarch(Buildarch)
 
 	fx.New(
 		fx.Provide(