docs: add a first-contribution quickstart transcript

[vishal2612200]

Goal

Add a short terminal transcript that shows a new contributor how to install AgentPack locally, run one focused test, and open a small PR.

Suggested files

• CONTRIBUTING.md
• docs/development.md

Acceptance criteria

• Shows setup with python -m pip install -e ".[dev]".
• Shows one narrow pytest command and one ruff command.
• Explains what to paste in a PR under Validation.
• Keeps the docs concise and avoids release-only workflow details.