PIPE-3252: add AGENTS.md and align ruff pre-commit
- Add AGENTS.md defining operational directives, architectural invariants, modern Python standards, and developer guidelines.
- Point .github/copilot-instructions.md to AGENTS.md as the canonical authority for AI assistant instructions.
- Switch .pre-commit-config.yaml to local hooks invoking pixi-managed ruff to guarantee parity across IDE, pre-commit, and agent workflows.
Pull request #2003: PIPE-3252: add link to CASA Docs citation guide in acknowledgements
Merge in PIPE/pipeline from PIPE-3252-add-casa-citation-page to main
* commit '8c6a6a273861cfa5da6e3f57ed6248f35169d7bb': PIPE-3252: add link to CASA Docs citation guide in acknowledgements
