fix(update): stop a timed-out git from leaving .git/index.lock behind

subprocess.run's timeout SIGKILLs only the direct git child, so the lock
is left behind and git's helpers are orphaned. On a slow SD card the same
pull then times out on every retry and recreates the lock, so removing it
by hand does not help.

- run_git starts git in its own session, stops the group with SIGTERM
  before SIGKILL, and clears the lock if nothing owns it. Update Code's
  pull, checkout, reset and stash go through it, with a 600s pull timeout.
- Read-only calls (describe --dirty, status, the update-check fetch) set
  GIT_OPTIONAL_LOCKS=0 so a timed-out poll cannot take the lock.
- A timed-out pull returns a clear error; a rebase left by an interrupted
  update is aborted before pulling.
- The manual-fix hint says to stop the services and kill running git.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

test/test_auto_update.py

@@ -823,6 +823,7 @@ def test_core_update_names_the_requirement_files_that_failed():
     pkg.api_v3.plugin_store_manager = None
     failed_install = subprocess.CompletedProcess([], 1, stdout='', stderr='boom')
     with patch.object(pkg.subprocess, 'run', run), \
+            patch.object(system.git_index_lock, 'run_git', run), \
             patch.object(system, '_pip_install_requirements', return_value=failed_install):
         result = system.perform_core_update()
     assert result['status'] == 'success'