docs: reorganize documentation into concepts, design, and examples
Move documentation files into a clearer structure:
- concepts/: core ideas (convoy, identity, molecules, polecat-lifecycle, propulsion)
- design/: architecture and protocols (architecture, escalation, federation, mail, etc.)
- examples/: demos and tutorials (hanoi-demo)
- overview.md: renamed from understanding-gas-town.md
Remove outdated/superseded docs and update reference.md.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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