v0.6.9: fix: reduce STEP_CARD_TOOL_ACTIVITY_PREVIEW_COUNT to 3 and update tests
Reduce the number of tool activity lines shown in step card previews from 5 to 3 for a more compact display. Update related tests to match the new preview count expectations. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>