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

CONVENTIONS.md

@@ -115,15 +115,13 @@ import (
 )
 
 var (
-    Appname   string = "CHANGEME"
-    Version   string
-    Buildarch string
+    Appname string = "CHANGEME"
+    Version string
 )
 
 func main() {
     globals.Appname = Appname
     globals.Version = Version
-    globals.Buildarch = Buildarch
 
     fx.New(
         fx.Provide(
@@ -823,7 +821,7 @@ 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,
     )
 }
 ```
@@ -943,23 +941,20 @@ import "go.uber.org/fx"
 
 // Package-level variables (set from main)
 var (
-    Appname   string
-    Version   string
-    Buildarch string
+    Appname string
+    Version string
 )
 
 // Struct for DI
 type Globals struct {
-    Appname   string
-    Version   string
-    Buildarch string
+    Appname string
+    Version string
 }
 
 func New(lc fx.Lifecycle) (*Globals, error) {
     n := &Globals{
-        Appname:   Appname,
-        Buildarch: Buildarch,
-        Version:   Version,
+        Appname: Appname,
+        Version: Version,
     }
     return n, nil
 }
@@ -970,15 +965,13 @@ func New(lc fx.Lifecycle) (*Globals, error) {
 ```go
 // cmd/httpd/main.go
 var (
-    Appname   string = "CHANGEME"  // Default, overridden by build
-    Version   string               // Set at build time
-    Buildarch string               // Set at build time
+    Appname string = "CHANGEME"  // Default, overridden by build
+    Version string               // Set at build time
 )
 
 func main() {
     globals.Appname = Appname
     globals.Version = Version
-    globals.Buildarch = Buildarch
     // ...
 }
 ```
@@ -989,10 +982,9 @@ Use ldflags to inject version information at build time:
 
 ```makefile
 VERSION := $(shell git describe --tags --always)
-BUILDARCH := $(shell go env GOARCH)
 
 build:
-    go build -ldflags "-X main.Version=$(VERSION) -X main.Buildarch=$(BUILDARCH)" ./cmd/httpd
+    go build -ldflags "-X main.Version=$(VERSION)" ./cmd/httpd
 ```
 
 ---