[FEATURE] Add gitPollingIntervalMs setting to reduce/disable background git polling

Question

### Problem

Claude Code polls git status every ~20 seconds per session. With many concurrent sessions in the same repo, this causes `.git/index.lock` contention and wasted IO on idle sessions.

### Proposal

```json
{ "gitPollingIntervalMs": 60000 }
```

Set to `0` to disable (on-demand only).

### Related

#57102, #11005