fix(cli): config set refuses to overwrite a corrupt config.json - #847

Summary

Closes #845.

`gitt config set` previously caught `json.JSONDecodeError` on the existing config, printed a yellow "starting fresh" warning, then wrote back a fresh dict containing only the new key — silently destroying every other configured value (`network`, `contract_address`, `ws_endpoint`, `hotkey`, …) on a single mid-write crash or on-disk corruption.

After this change the JSONDecodeError branch raises `SystemExit(1)` with a clear error pointing at the file, leaving the corrupt bytes on disk so the operator can inspect or remove them before re-running. Same refuse-to-overwrite invariant as #781 (PATs file) and the read-side fix in #817 (`load_config` + `_load_config_value`).

```
$ gitt config set wallet alice
Error: Existing config file at /home/me/.gittensor/config.json is not valid JSON (Expecting value: line 1 column 1 (char 0)).
Refusing to overwrite — inspect or remove the file before re-running.
$ echo $?
1
```

Related Issues

Closes #845. Same family as #781 (write side, PATs) and #817 (read side, this same file).

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• New `tests/cli/test_config_set_corruption.py` covers three cases:
  • corrupt existing JSON → exit 1, file untouched on disk (the critical invariant — the corrupt bytes must remain intact for inspection).
  • valid existing JSON → existing keys preserved when appending a new key.
  • missing file → fresh single-key config written.
• `uv run --extra dev pytest tests/` — 646 tests pass (643 baseline + 3 new).

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)