Add the project design document (`docs/explanation/design.md`, rationale + decision history) and the distilled technical specification (`docs/reference/tech-spec.md`, the build artifact) defining hephaestus as a unified, self-hosted, **client/server + offline-first** task + knowledge-base system: typed node graph, the lived priority discipline ("what is next?"), recurrence with fresh-per-occurrence checklists, op-log/CRDT sync with conflict resolution, OIDC/Authentik auth, the heph.nvim surface, and a test-driven development strategy.