Unify agent save field with insight output field
Replace agent-specific "save" frontmatter (full filename) with unified
"output" field (format only: "md" or "json"), matching insight behavior.
Output paths are now derived from persona name and schedule context.
- Add shared get_output_path() to think/utils.py for DRY path derivation
- Update think/insight.py to use shared function
- Rename _save_agent_result to _write_output in cortex.py
- Pass segment in config from dream.py for segment agent path resolution
- Remove unused save parameter from cortex_client.py
- Update decisionalizer.md frontmatter and CORTEX.md docs
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>