feat(integrations): support automatic branch cleanup after merge

Add a new git::review_cleanup module that implements a CleanupPlan with
prepare() and finish() routines to safely remove/clean tracking and local
branches after a PR/MR is merged. The cleanup logic validates branch names,
ensures a clean worktree, checks remotes/URLs, verifies commits/ancestry,
protects against concurrent worktrees or divergent local/remote commits, and
performs authenticated fetch/push and ref updates. Unit tests for the cleanup
behavior are included.

Wire provider-side cleanup into integrations:
- add an integrations/cleanup module to read provider PR payloads and derive
  cleanup inputs
- run cleanup::prepare(...) before performing a merge when an optional
  cleanup_path is provided
- after a successful provider merge, run cleanup::finish(...); any failure is
  reported as MERGE_ACCEPTED_CLEANUP_FAILED

Also:
- export the new git review_cleanup module (src-tauri/src/git.rs)
- accept an optional cleanup_path parameter in run_integration_review_action
- remove the previous REVIEW_REQUEST_TIMEOUT wrapper around the spawned
  blocking task (the integration action is no longer wrapped with the 35s timeout)

src/App.svelte

@@ -6117,6 +6117,14 @@
         localResolutionMessage={reviewConflictMessage}
         loadCredential={loadStoredCredential}
         onOpenSettings={() => { appSettingsOpen = true; }}
+        onCleanupStateChange={async (busy, path) => {
+          if (busy) {
+            operation = appLanguage === "de" ? "PR zusammenführen und Branch aufräumen" : "Merging PR and cleaning up branch";
+          } else {
+            try { if (sameRepoPath(activeRepoPath, path)) await refreshRepositorySnapshot(path); }
+            finally { operation = ""; }
+          }
+        }}
         onStartLocalResolution={startReviewConflictResolution}
         onOpenLocalResolver={reopenReviewConflictResolver}
         onContinueLocalResolution={continueReviewConflictMerge}