[Ticket]: de-duplicate checkpoint()/field() across eight copilot_push tests

## Summary

Seven `copilot_push_*` integration tests each carry their own byte-identical copy of the same two helpers, `checkpoint()` and `field()`.

## Affected

```
tests/copilot_push_flaky.rs tests/copilot_push_partial.rs
tests/copilot_push_rotation.rs tests/copilot_push_held.rs
tests/copilot_push_interrupted.rs tests/copilot_push_invariants.rs
tests/copilot_push_loss.rs (+ copilot_push_quarantine.rs, split from flaky)
```

## Why it was not fixed in #228

The duplication was noticed while splitting `copilot_push_flaky.rs` to satisfy the LoC gate. Lifting the helpers into `tests/support/` touches eight files, which does not belong in a gate fix on an already-large branch. Following the existing convention was the smaller, more reviewable choice at the time.

## Scope

Move both helpers into `tests/support/` (a small `checkpoint.rs` module beside `copilot.rs`, exported from `support/mod.rs`) and delete the copies. Purely mechanical; no assertion should change.

Note `support/copilot.rs` is at 196 lines against the 200 ceiling, so the helpers need their own module rather than being appended there.

## AI Usage Declaration

AI-assisted. File list produced by grep, not recalled. A human owns intent, verification and consequences.