Fix reputation scoring validator dead loop

Description

The pitch.

Fixes #98

## Summary

Fixes the reputation scoring validator so it enforces a concrete invariant instead of accumulating scores and leaving the validation loop empty.

## Changes

- Distinguishes explicit `cheat:` events from agents who merely receive a bad report
- Requires every explicit cheating event to have a corresponding bad reputation report
- Removes the dead score-tracking loop
- Adds a regression test proving that a bad report alone does not count as cheating

## Testing

- `uv run pytest packages/nest-core/tests/test_validators.py::TestReputationScoring -v`
- 4 tests passed