chore: Use DEFAULT_DATASET_ITEMS_LIMIT in get-actor-run nextStep templates

[jirispilka]

Problem

After #814 lands, get-dataset-items will default limit to 20 via a file-local DEFAULT_DATASET_ITEMS_LIMIT constant. The nextStep templates in src/tools/core/actor_run_response.ts still hardcode limit=20 as a magic number in three places (lines ~244, 253, 285). If the default ever changes, the nextStep hints drift silently.

Proposed solution

1. Export DEFAULT_DATASET_ITEMS_LIMIT from src/tools/common/get_dataset_items.ts.
2. Import it in actor_run_response.ts and interpolate into the three nextStep templates instead of the literal 20.

Blocked by: #814 (don't start until it merges).