runner: hide PowerShell window + filter PS error noise from log

- HideWindow: true in SysProcAttr (Windows) prevents powershell.exe
  from opening a console window over the GUI
- skipPSNoiseLine filters multi-line PS error blocks (At line:, CategoryInfo,
  FullyQualifiedErrorId, VERBOSE: etc.) - errors still appear via Write-Log

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

internal/runner/hidecmd_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package runner
+
+import "os/exec"
+
+func hideWindow(cmd *exec.Cmd) {}

internal/runner/hidecmd_windows.go

@@ -0,0 +1,14 @@
+//go:build windows
+
+package runner
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// hideWindow prevents the child process from creating a visible console window.
+// Without this, powershell.exe opens a full-size window on top of the GUI.
+func hideWindow(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
+}

internal/runner/runner.go

@@ -223,6 +223,7 @@ func (r *Runner) runScript(ctx context.Context, step Step, cfgArg string) error
 	}
 
 	cmd := exec.CommandContext(ctx, "powershell.exe", args...)
+	hideWindow(cmd) // prevent PS console window from appearing over the GUI
 
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
@@ -237,6 +238,9 @@ func (r *Runner) runScript(ctx context.Context, step Step, cfgArg string) error
 	scanner := bufio.NewScanner(stdout)
 	for scanner.Scan() {
 		line := scanner.Text()
+		if skipPSNoiseLine(line) {
+			continue
+		}
 		r.onLog(LogLine{
 			StepID: step.ID,
 			Text:   line,
@@ -247,6 +251,33 @@ func (r *Runner) runScript(ctx context.Context, step Step, cfgArg string) error
 	return cmd.Wait()
 }
 
+// skipPSNoiseLine returns true for PowerShell stderr noise that clutters the log:
+// multi-line error blocks (At line:N, CategoryInfo, FullyQualifiedErrorId, etc.),
+// blank lines, and VERBOSE: prefix lines already handled by Write-Log.
+func skipPSNoiseLine(line string) bool {
+	trimmed := strings.TrimSpace(line)
+	if trimmed == "" {
+		return true
+	}
+	for _, prefix := range []string{
+		"At line:",
+		"+ CategoryInfo",
+		"+ FullyQualifiedErrorId",
+		"+ PositionMessage",
+		"VERBOSE:",
+		"DEBUG:",
+	} {
+		if strings.HasPrefix(trimmed, prefix) {
+			return true
+		}
+	}
+	// PS error continuation lines start with spaces + "+" or "~"
+	if len(trimmed) > 0 && (trimmed[0] == '+' || strings.HasPrefix(trimmed, "~")) {
+		return true
+	}
+	return false
+}
+
 // parseLevel extracts the log level from lines formatted as "[HH:mm:ss] [LEVEL] message".
 func parseLevel(line string) string {
 	if strings.Contains(line, "] [OK]") {