Simplify README and organize technical documentation

Replace the long root README with a plain-language system overview, quick local/development commands, and a short production-considerations section. Preserve useful technical detail in docs/: architecture/history, foundation-only database diagram and regeneration, development/logging/database safety, prototype API, and frontend design. Keep security limitations and synthetic-only development constraints clear. Update moved-file references; no application changes or commits. Acceptance: README is substantially shorter and usable by a newcomer; detailed guides and diagram are discoverable under docs/; repository-relative links resolve and documentation commands agree with existing tooling. Verified: README reduced from 197 to 73 lines (2089 to 557 words); architecture/diagram explanation, development/logging/safety, prototype API, and relocated frontend guide live in docs/. Updated AGENTS.md's frontend-guide reference. All 30 relative documentation links and anchors resolve; documentation whitespace and git diff --check pass. Command references checked against dev/run.py. Application/browser suites not run for this documentation-only change; diagram artifacts unchanged. No commits.