docs(agency): add agency-framework.md and update README (WP-0002 T15-T16)
- Add docs/agency-framework.md: full explanation of the project memory
model, session protocols, memory frontmatter field, CLI reference,
coach meta-agent usage, and protocols preview (Part 3)
- Update README: reposition as agency framework (not just agent library),
add Agency Framework section with memory CLI examples, update feature
list to 18 agents, add sys-medic and coach to agent listing
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>