Fix: docs: add root-level README.md

[brite-side0]

Closes #1667

Summary

The file currently only contains a 'Handsoff notes' section with issue references; the new README content should be added above this section or the section retained at the bottom. Verify that all referenced files (docs/architecture.md, docs/data-flow.md, CONTRIBUTING.md, API_SPEC.md, docs/deployment.md, SECURITY.md) actually exist in the repository before linking. Ensure the architecture summary matches the current codebase structure.

Changes

• Create a new root-level README.md that includes: a one-paragraph project summary of predictIQ; a high-level architecture section with a textual summary and links to docs/architecture.md and docs/data-flow.md; a repository layout overview covering services/, contracts/, frontend/, infrastructure/, and docs/; a local development quickstart section or a link to CONTRIBUTING.md; and links to API_SPEC.md, docs/deployment.md, and SECURITY.md. Preserve the existing 'Handsoff notes' section at the bottom of the file. (README.md)