docs: add SDD specs + user requirements + tests
- .agents/specs/: spec-driven-dev docs for arcrun MVP, auth-recipe,
credential-primitives-wasm (active refactor), landing-page,
sdk-and-website, u6u-core-mvp, u6u-platform-evolution.
- .agents/steerings/tech.md: detailed tech stack rationale.
- docs/user_requirements/: long-form requirements incl. credential
primitives, pages spec, py strategy analysis.
- tests/: end-to-end harness scaffolding.
These are the durable context backing CLAUDE.md's SDD protocol.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Leo
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