fix: Code review fixes for GoReleaser setup

- Change version.go constants to variables for ldflags to work
- Fix changelog regex to properly match feat(scope): and fix(scope):
- Enable windows/arm64 builds (pure Go, no CGO issues)
- Add concurrency guard to release workflow

Oracle review feedback implemented.

cmd/bd/version.go

@@ -9,8 +9,8 @@ import (
 	"github.com/steveyegge/beads/internal/rpc"
 )
 
-const (
-	// Version is the current version of bd
+var (
+	// Version is the current version of bd (overridden by ldflags at build time)
 	Version = "0.15.0"
 	// Build can be set via ldflags at compile time
 	Build = "dev"