Convert to using modern Python packaging infrastructure

[Thomas Hopkins (thopkins32)]

Generated-by: opencode:gpt-5.5 and reviewed by me.

Migrated the repo to modern packaging/tooling:

• Added pyproject.toml with Hatchling + Hatch-VCS, project metadata, scripts, dependencies, Ruff config, and pytest config.
• Added sync-experiment extra: pip install -e ".[sync-experiment]". would break existing deployments and complicate conda-forge packaging.
• Moved dev tooling into dev extra: pip install -e ".[dev]".
• Kept PyYAML in core because nslsii.kafka_utils also uses it.
• Replaced Versioneer with Hatch-VCS version generation.
• Removed legacy packaging files: setup.py, setup.cfg, requirements*.txt, pytest.ini, .flake8, MANIFEST.in, versioneer.py, tracked nslsii/_version.py.
• Removed stale Travis files and deploy key artifact: .travis.yml, github_deploy_key_nsls_ii_nslsii.enc.
• Updated Pixi with dev, py310, py311, py312 environments and tasks.
• Updated pre-commit to run Ruff format then Ruff lint/fix.
• Updated GitHub Actions to install .[dev], run pre-commit, and build via python -m build.
• Updated AGENTS.md for the new workflow.
• Ran Ruff formatting across the repo.

Probably shouldn't be merged until recent PRs have been merged first. The formatting across the repository will cause rebasing/merging to be difficult.

Closes #206

[Thomas Hopkins (thopkins32)]

Will split this up into smaller PRs