restic: RunPrune + runWithPump helper, refactor Forget/Init onto it

Add RunPrune for admin-credential prune invocations.  Extract
runWithPump to DRY the stdout+stderr pump pattern; refactor RunForget
and RunInit to delegate to it (RunInit preserves the "config file
already exists" soft-success sniff by wrapping the handler before the
call).  Add runner_test.go with TestRunPruneInvokesPrune.

internal/restic/runner_test.go

@@ -0,0 +1,53 @@
+package restic
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// setupScriptBin writes a small shell script to a temp directory,
+// makes it executable, and returns its path. scriptBody is the
+// complete script content (without the shebang line — that's added
+// automatically).
+func setupScriptBin(t *testing.T, scriptBody string) string {
+	t.Helper()
+	dir := t.TempDir()
+	p := filepath.Join(dir, "restic")
+	content := "#!/bin/sh\n" + scriptBody + "\n"
+	if err := os.WriteFile(p, []byte(content), 0o755); err != nil {
+		t.Fatalf("setupScriptBin: %v", err)
+	}
+	return p
+}
+
+// captureLines returns a LineHandler that appends "stream:line" into
+// the returned slice pointer (safe for single-goroutine test use).
+func captureLines() (*[]string, LineHandler) {
+	var lines []string
+	h := func(stream, line string, _ any) {
+		lines = append(lines, fmt.Sprintf("%s:%s", stream, line))
+	}
+	return &lines, h
+}
+
+// --- B1: RunPrune ---
+
+func TestRunPruneInvokesPrune(t *testing.T) {
+	// Shell script that echoes its args; "prune" should appear in output.
+	bin := setupScriptBin(t, `echo "$@"`)
+	env := Env{Bin: bin}
+	lines, h := captureLines()
+	if err := env.RunPrune(context.Background(), h); err != nil {
+		t.Fatalf("RunPrune returned error: %v", err)
+	}
+	for _, l := range *lines {
+		if strings.Contains(l, "prune") {
+			return
+		}
+	}
+	t.Fatalf("expected 'prune' in captured output; got: %v", *lines)
+}