fix(engine): sequence-loop detector requires stagnant repeats - #55

## What

Consecutive repetition of a step-name pattern now counts as a loop only when the repeated steps are stagnant (identical inputs and outputs). Repetition with different inputs is treated as legitimate iteration.

## Why

Found by our own live gate on the public demo repo. The demo agent queries three states, producing `decide -> call -> decide -> call -> decide -> call`. The detector flagged that as a loop and the gate hard-blocked the run with `Blocked by: loops` even though trajectory divergence was `0.0` and every call had different arguments. That shape is the single most common agent pattern, so this was a false positive that would hit real users immediately.

## How

`detect_sequence_loops` already computed a `stagnant` flag and then ignored it. The loop is now appended only when `stagnant` is true. Runaway loops with drifting args stay covered by `max_tool_repeats` and the token/step bands, and the identical-call invariant is unchanged.

## Testing

- make lint -> All checks passed
- uv run python -m pytest -> 437 passed (5 new tests)

New tests: iteration over items is not a loop; repeats with changed outputs are not a loop; stagnant repeats still are.