fix(sat): kill entire process group on task cancel

exec.CommandContext only kills the direct child (the shell script), leaving
grandchildren (john, gpu-burn, etc.) as orphans. Set Setpgid so each SAT
job runs in its own process group, then send SIGKILL to the whole group
(-pgid) in the Cancel hook.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/platform/sat.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"syscall"
 	"sort"
 	"strconv"
 	"strings"
@@ -531,6 +532,13 @@ func runSATCommandCtx(ctx context.Context, verboseLog, name string, cmd []string
 	}
 
 	c := exec.CommandContext(ctx, resolvedCmd[0], resolvedCmd[1:]...)
+	c.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	c.Cancel = func() error {
+		if c.Process != nil {
+			_ = syscall.Kill(-c.Process.Pid, syscall.SIGKILL)
+		}
+		return nil
+	}
 	if len(env) > 0 {
 		c.Env = append(os.Environ(), env...)
 	}