feat: add auto parallelism mode

[MilesCranmerBot]

Closes #1018.

Adds parallelism="auto" while keeping the constructor default as None for backward-compatible signatures/get_params behavior. None now resolves like auto:

• deterministic=True -> serial
• procs or cluster_manager set -> multiprocessing
• otherwise -> the existing multithreading default

Explicit serial, multithreading, multiprocessing, and deprecated multithreading= behavior are preserved.

Validation:

• targeted unit test passed: uv run --python /usr/bin/python3.12 --extra dev pytest pysr/test/test_main.py::TestHelpMessages::test_parallelism_auto -q
• coverage check verified all changed executable/doc lines in pysr/sr.py are covered
• git diff --check passed
• ponytail review passed

[MilesCranmer]

@MilesCranmerBot I think this is the wrong location, it should be done in the Julia side. PySR should just forward the parallelism mode but not have any moving pieces. Please close + make a SR.jl PR instead + make a PySR PR to update compatibility with the backend change.