Add docs and AGENTS guidance

[jdfalk]

Summary

• document contributor guidelines in AGENTS.md
• add initial CHANGELOG.md and TODO.md
• expand README with features, usage and development notes

Testing

• go test ./...

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https://chatgpt.com/codex/tasks/task_e_6843717070cc8321ac677fbce7fa7949

[github-actions]

Testing infrastructure is still needed. Current status:

❌ No pytest-based unit tests for issue_manager.py
❌ No workflow testing framework
❌ No integration tests

This aligns with TODO.md Phase 3: Testing and Quality. Should remain open.

[github-actions]

Testing infrastructure is still needed. Current status:

❌ No pytest-based unit tests for issue_manager.py
❌ No workflow testing framework
❌ No integration tests

This aligns with TODO.md Phase 3: Testing and Quality. Should remain open.