[WIP] Add auto_ncl to VaspJob

[mkhorton]

Allows Custodian to more easily work with non-collinear VASP.

Opening in draft, still WIP, needs tests.

[Andrew-S-Rosen]

@mkhorton: I think this is very smart to do, seeing as VASP ships it this way (and not with .gamma). I don't see the change made to the source code though, so if that's the case, just a head's up!

[Andrew-S-Rosen]

VASP ships as vasp_ncl so that should probably be what is used.

[shyuep]

Automated PR review generated by Claude (Opus 4.7) on behalf of @shyuep. Based purely on the diff and current GitHub Actions state — no code was executed locally.

Small, sensible addition that mirrors the existing auto_gamma pattern for non-collinear runs. CI (lint + all build matrix jobs) is green on the head commit. A few items before un-drafting:

• ⚠️ Behavior change from the new default auto_ncl=True. Existing users with an LNONCOLLINEAR/LSORBIT INCAR and a vasp_ncl (or vasp_std.ncl) in PATH will silently switch executables on the next custodian run. For a 1.0-style stable utility, prefer auto_ncl=False and let users opt in, or at minimum call this out in the changelog.
• ⚠️ cmd[-1].replace("std", "ncl") is brittle. If cmd[-1] doesn't contain "std" (e.g. vasp, vasp_gpu, or a path like /opt/vasp/std/vasp_std where "std" appears in a parent dir), the replace either no-ops (re-checks the same binary) or rewrites the path segment. Safer to gate on "std" in cmd[-1] before falling into that branch, or use a regex anchored to the basename.
• ⚠️ Echoes Andrew's Aug 2025 comment: VASP's own build ships as vasp_ncl (analogous to vasp_std/vasp_gam), so the std→ncl swap is the realistic path for most users; the .ncl-suffix branch is mostly cosmetic. Consider documenting vasp_ncl as the canonical name.
• ⚠️ Docstring drift for auto_gamma: the diff updates the auto_gamma docstring to advertise "gamma" replacing "std" behavior, but the surrounding run() block only implements the .gamma suffix — there is no std→gamma substitution. Either add that branch (parallel to the new auto_ncl logic) or revert the docstring change.
• ℹ️ Tests: PR body already flags "needs tests." A unit test that monkeypatches shutil.which to assert each of the three resolution paths (ncl_vasp_cmd provided, .ncl suffix exists, std→ncl substitution exists, and the fall-through with no match) would be sufficient.