Expand contributor docs and GitHub templates

[huazhuang80-star]

Why

Contributors currently only see the scaffold placeholder in CONTRIBUTING.md, and the repository does not have issue forms or a PR template that prompt for setup, affected layer, test evidence, or contract impact.

Closes #19

What changed

• Replaced the placeholder CONTRIBUTING.md with prerequisites, local setup, frontend/backend/contract commands, quality checks, style guidance, conventional commits, and a PR checklist.
• Added bug report, feature request, and contract issue forms under .github/ISSUE_TEMPLATE/.
• Added .github/pull_request_template.md with test evidence, contract impact, and safety checklist sections.

Type of change

• docs
• dx
• chore / ci

Test evidence

• Verified all changed files are present on the fork via the GitHub contents API.
• npm run lint
• npx prettier . --check
• npm run build
• npm test --if-present
• cd backend && npm run build
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --locked
• STELLAR_SCAFFOLD_ENV=development stellar-scaffold build --build-clients
• npm run install:contracts
• Not run; docs/template-only change.

Contract impact

• No contract changes
• Affected functions/modules are listed
• Events, storage layout, generated clients, and security implications are documented

Safety checklist

• No secrets, .env files, private keys, generated credentials, or build artifacts are committed
• User-visible behavior, API changes, or environment variable changes are documented
• Follow-up work is explicitly listed