docs: add AGENTS.md and PR template

[avoidwork]

Summary

Add AGENTS.md with rules and principles for agents working on the project, including coding conventions, security rules, git workflow, and operational guidance. Also adds a PR template at .github/PULL_REQUEST_TEMPLATE.md to standardize pull request descriptions.

Related

N/A

Changes

• Replaces brief Haro project guide with comprehensive AGENTS.md covering Python conventions, FastAPI/LangGraph framework rules, git workflow, and operational learnings
• Adds .github/PULL_REQUEST_TEMPLATE.md with structured sections for PR submissions

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the CONTRIBUTING document.
• My code follows the style guidelines (pylint, isort, black, mypy).
• I have added tests that cover my changes.
• All new and existing tests passed.
• Commit message follows Conventional Commits.
• No hardcoded secrets or credentials introduced.
• Threat model considerations addressed (if applicable).