Add CONTRIBUTING.md with developer onboarding guide

[Wilfred007]

What

CONTRIBUTING.md is essentially empty (3 lines). Add comprehensive contributing guide with setup instructions, code standards, PR process, and architecture overview.

Why

New contributors have no guidance on how to set up the project, code standards, or PR process. This blocks community contributions.

Scope

In scope: Setup instructions, code standards, PR template, architecture overview, testing guide
Out of scope: Governance model

Acceptance Criteria

• Local development setup (frontend, backend, contracts)
• Code style and standards
• PR process and review criteria
• Architecture overview with diagrams
• Testing guide

Technical Context

• CONTRIBUTING.md — current 3-line placeholder
• README.md — basic setup instructions