Add integration blackbox tests for g3-core
Adds 18 new integration tests covering:
- Background process lifecycle (start, check running, kill, list)
- Unified diff edge cases (multi-hunk, additions-only, deletions-only,
CRLF normalization, range constraints, error handling)
- Error classification boundaries (rate limit, server error, timeout,
network error, context length exceeded, model busy, non-recoverable)
These tests follow blackbox/integration-first principles:
- Test through stable public interfaces
- Do not encode internal implementation details
- Focus on observable behavior
- Enable refactoring without test breakage
Agent: hopper
83c9b5d434