#83 FTX-83: Repo docs — README + docs/ (architecture, sequence, flow) + CONTRIBUTING

EARS SPEC: - The repository shall provide a short, effective README.md that states what Futex is, links to the /claude agent guide and live /docs, and links to each docs/*.md. - The repository shall provide docs/ covering architecture (components + package layout + data stores), the data model, the decision/task state machine, and request/approval sequence + flow diagrams. - Where a diagram is included, the docs shall use Mermaid so it renders natively on GitHub. - The repository shall provide CONTRIBUTING.md covering dev setup, running tests, the migretti migration rule, the EARS/issuedb workflow, and commit/PR conventions. - The documentation shall describe only behavior that exists in the codebase (no aspirational claims).