[documentation] Add CONTRIBUTING.md with Wave bounty label workflow

[yinkscss]

Context

WaveFlow targets Stellar Wave open-source contributors. Drips Wave maintainers assign complexity via labels (Stellar Wave, complexity tiers).

Problem

Repo lacks CONTRIBUTING.md explaining how to pick issues, label conventions, and Wave program alignment (PRD NG6: no Drips API dependency).

Proposed scope

• Add CONTRIBUTING.md with branch/PR conventions
• Explain Wave label workflow mapped to gateway point mapping issue
• Link PRD, ROADMAP, AGENTS.md, and core-loop doc

Acceptance criteria

• CONTRIBUTING.md exists at repo root
• Documents issue label prefixes ([gateway], [api], etc.)
• Explains WaveFlow is Stellar-native (PRD NG6)
• README links to CONTRIBUTING.md

References

• README.md
• AGENTS.md
• docs/PRD.md NG6
• Drips Wave maintainer docs