Integrate / Merge / Copy between branches

- Tools -> Integrate / Merge / Copy: pick op + source/target paths, runs into a
  new pending changelist; resolve (if needed) via the Resolve UI and submit
- Backend p4 merge/copy/integrate -c

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/p4.ts

@@ -67,6 +67,7 @@ export const p4 = {
   syncTo: (scope: string, rev: string) => invoke<string>("p4_sync_to", { scope, rev }),
   labels: () => invoke<{ label?: string; Update?: string; Owner?: string; Description?: string; [k: string]: unknown }[]>("p4_labels"),
   labelTag: (label: string, scope: string, rev = "") => invoke<string>("p4_label_tag", { label, scope, rev }),
+  branchOp: (op: "merge" | "copy" | "integrate", source: string, target: string) => invoke<string>("p4_branch_op", { op, source, target }),
   diff: (file: string) => invoke<string>("p4_diff", { file }),
   revert: (files: string[]) => invoke<string>("p4_revert", { files }),
   submit: (description: string, files: string[]) =>