refactor(cli): drop vars that only kept imports alive

exec.go held `var _ = json.Marshal` and `var _ = strings.Join` to keep
imports in use; json had no other use, so its import goes too.
TestE2E_BuildUpExec_Image built an exec.Cmd it never ran.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: aledbf

internal/cli/e2e_test.go

@@ -37,9 +37,6 @@ func TestE2E_BuildUpExec_Image(t *testing.T) {
 		t.Fatal("no containerId in up output")
 	}
 
-	cmd := exec.Command(os.Args[0], "-test.run=^$")
-	_ = cmd
-
 	execCmd := exec.Command("docker", "exec", containerID, "echo", "e2e-test-ok")
 	execOut, err := execCmd.CombinedOutput()
 	if err != nil {

internal/cli/exec.go

@@ -3,7 +3,6 @@ package cli
 import (
 	"cmp"
 	"context"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -437,6 +436,3 @@ func splitExecArgs(args []string) (flags []string, cmd []string) {
 	}
 	return flags, nil
 }
-
-var _ = json.Marshal
-var _ = strings.Join