Flesh out CONTRIBUTING.md with a real development setup guide

[nojibe]

Problem

CONTRIBUTING.md's "Development Setup" section is literally "Coming soon." New contributors have no documented path to run the app or the eval CLI locally, despite a non-trivial setup (pnpm, Node pinned in .nvmrc, a ~2.7KB .env.template of provider keys and service config, Next.js dev server on port 3172, and separate web/CLI test suites).

Proposed change

Write a real Development Setup section (or a docs/DEVELOPMENT.md that CONTRIBUTING links to) covering:

• Prerequisites: Node version (.nvmrc), pnpm@9.6.0 (from packageManager)
• pnpm install
• Copying .env.template → .env and which vars are required vs optional to run locally
• Running the app: pnpm dev (port 3172)
• Running the CLI: pnpm cli ...
• Running tests: pnpm test:web, pnpm test:cli, pnpm test:e2e
• Typecheck/lint before pushing; note the husky/lint-staged pre-commit hook
• How to author and validate a blueprint locally (link docs/BLUEPRINT_FORMAT.md)

Acceptance criteria

• A contributor can clone → configure → run the app and tests from the docs alone
• Required vs optional env vars are clearly distinguished
• "Coming soon." placeholder is gone