feat: add no-push config to disable automatic git push

Wire up the existing --no-push flag to a config option so it can be
set as the default, and update bd prime output accordingly.

- Add no-push default to config.go, matching existing pattern
- Check config in sync.go when --no-push flag not explicitly set
- Update bd prime output to omit git push step when enabled

docs/CONFIG.md

@@ -34,6 +34,7 @@ Tool-level settings you can configure:
 | `no-daemon` | `--no-daemon` | `BD_NO_DAEMON` | `false` | Force direct mode, bypass daemon |
 | `no-auto-flush` | `--no-auto-flush` | `BD_NO_AUTO_FLUSH` | `false` | Disable auto JSONL export |
 | `no-auto-import` | `--no-auto-import` | `BD_NO_AUTO_IMPORT` | `false` | Disable auto JSONL import |
+| `no-push` | `--no-push` | `BD_NO_PUSH` | `false` | Skip pushing to remote in bd sync |
 | `db` | `--db` | `BD_DB` | (auto-discover) | Database path |
 | `actor` | `--actor` | `BD_ACTOR` | `$USER` | Actor name for audit trail |
 | `flush-debounce` | - | `BEADS_FLUSH_DEBOUNCE` | `5s` | Debounce time for auto-flush |