fix: refine PR template with project-specific commands, env vars, and checklist

[Xuccessor]

Summary

The existing .github/pull_request_template.md lacked project-specific context (commands, env vars). This PR refines the template so contributors run the project's make lint / make test flow and surface any environment-variable changes.

Changes

• Added a Commands Run hint in Testing Done that references make ci and poetry run pytest web_app/tests -v --tb=short for backend changes.
• Added an Environment Variables section with a .env.example hint so contributors update deployment docs alongside code.
• Tightened the checklist (make lint, make test, CI green, env vars, issue link).
• Removed the bureaucratic "Rollback Plan" section — keeps the template lean.

Testing

• Manually rendered the markdown to confirm it stays readable and scales for typical PR sizes.
• Cross-checked against Makefile, quantara/pyproject.toml, and .github/workflows/ci.yml to make sure the referenced commands actually exist.

Closes #71