[DX] Expand CONTRIBUTING.md — full setup guide, PR checklist, issue templates #18

Overview

The existing CONTRIBUTING.md is minimal. For an open-source project post-hackathon, it needs to be complete enough that a developer can go from zero to PR without asking for help.

What needs to be done

1. Expand CONTRIBUTING.md

• Prerequisites with install commands (Rust, Node, Stellar CLI, Scaffold plugin)
• Full local setup step-by-step
• Running each part independently (contract / frontend / backend)
• Running the test suite for each layer
• Code style guide (ESLint/Prettier for TS, `cargo fmt` + `cargo clippy` for Rust)
• PR checklist: tests added, lint passes, contract tests pass, explains why not what
• Commit message format (conventional commits)

2. Issue templates (`.github/ISSUE_TEMPLATE/`)

• `bug_report.yml`
• `feature_request.yml`
• `contract_issue.yml` — affected function, security implications

3. PR template (`.github/pull_request_template.md`)

Acceptance criteria

• New contributor can set up and run using only CONTRIBUTING.md
• Issue templates render on GitHub
• PR template prompts for test evidence