Make datetime context explicit via instructions.now and instructions.day
Replace implicit include_datetime behavior with explicit frontmatter settings:
- instructions.now: Include current datetime in extra_context
- instructions.day: Include analysis day context (requires day parameter)
Key changes:
- Add format_current_datetime() helper in prompts.py as single source
- Update compose_instructions() and get_agent() to use analysis_day param
- Change template vars from $date to $day/$day_YYYYMMDD for consistency
- Add $now template variable for current datetime
- Update agent prose from "yesterday" to "the analysis day" for clarity
- Update docs to reflect new template variables
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>