activity_state: add active_entities field to activity output
Prompt now asks LLM to include an active_entities array of people,
companies, projects, and tools noticeably active per activity.
Post-hook passes through on active items, omits on ended. Bumped
max_output_tokens to 1024 to accommodate.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>