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.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,
 	)
 }