fix: address final-review findings before closing Plan 1

- privilege: wire sudo -v to real stdin/stdout/stderr so it can prompt
- cmd: validate --only/--skip IDs against registry (prevents silent
  run-all on typo); gate install to Arch-only (spec requirement);
  mark --only and --skip mutually exclusive
- platform: Pacman.Update now runs -Syu --noconfirm, not -Sy (avoids
  Arch partial-upgrade anti-pattern)
- test/integration: trap-clean the named docker volume on any exit
- go.mod: tidy — yaml.v3 is a direct dep

internal/privilege/sudo.go

@@ -5,6 +5,7 @@ package privilege
 import (
 	"context"
 	"fmt"
+	"os"
 	"os/exec"
 	"time"
 )
@@ -36,7 +37,11 @@ type ExecSudo struct{}
 
 func (ExecSudo) Validate(ctx context.Context) error {
 	cmd := exec.CommandContext(ctx, "sudo", "-v")
-	cmd.Stdin = nil // inherit terminal stdin
+	// Connect to the real terminal so sudo can prompt for the password.
+	// exec.Cmd.Stdin defaults to /dev/null, not the parent process's stdin.
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
 	return cmd.Run()
 }