docs(readme): finalize repo README, diagrams, and developer quickstart

[thopatevijay]

• Use the README created earlier. Save final architecture SVGs into /designs.

• Add docs/Project_Proposal.pdf and docs/User_Stories.pdf placeholders.

• Add Quickstart section showing npx @castui/cli and how to generate a UI.

Acceptance criteria

• README contains Quickstart and diagrams link.

• designs/*.svg present in repo.