refactor: move context injection into dynamic system prompts
use pydantic-ai's @agent.system_prompt(dynamic=True) to inject
date, thread, memory, episodic, and reflection context as system
prompts instead of stuffing everything into the user prompt.
simplifies process_mention and process_reflection significantly.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>