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-tauri/src/git.rs

@@ -1,4 +1,5 @@
 pub mod submodules;
+pub(crate) mod review_cleanup;
 use serde::{Deserialize, Serialize};
 use std::{
     collections::{BTreeMap, BTreeSet},