Async spawn for hot git IPC paths (diff, listFiles, repo info) #359

From docs/ISSUES.md (issue #88 remainder, explicitly 'not taken'): sync git calls on hot IPC paths are bounded at 15s but still block the Electron main process — and agent streaming with them — for the whole bound. This is the last unbounded-latency main-process freeze class after #88. The #325 overhead benchmark will surface it; fix is async spawn.