Config Corruption Auto-Repair (GH #3117)

What changed: When ~/.claude.json is found corrupted during a locked save operation, Claude Code now automatically repairs it from the in-memory cached config rather than leaving the file in a corrupted state.

Previously, hitting a parse error on the re-read inside saveConfigWithLock would block the write and leave the file unchanged. Now:

1. If a parse error is detected during the locked re-read, the cached in-memory config is used as the recovery source.
2. The repair is logged and reported to telemetry (tengu_config_auto_repaired).
3. A timestamped .corrupted. backup is still saved so the original can be inspected.
4. The error message changed from "Config file corrupted, resetting to defaults" to "Config file corrupted" and separately "Could not back up corrupted config" for backup failures — more accurate since defaults are no longer the only fallback.