Consolidate CmdStan path resolution

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

(PR summary generated by codex)

This PR follows up on the WSL CI fix by consolidating CmdStan path resolution and cleaning up a few related path helpers.

• Routes package startup through set_cmdstan_path() so startup and explicit set_cmdstan_path() calls use the same resolution logic.
• Centralizes CMDSTAN handling in resolve_cmdstan_path_from_env().
• Clears cached CmdStan path/version/WSL state consistently for explicit invalid CMDSTAN and unsupported versions
• Removes the legacy auto-migration of an unversioned cmdstan/ directory to cmdstan-<version>.
• Updates cmdstan_make_local() to read make/local without using file.info(), which avoids fragile file metadata calls.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.21%. Comparing base (2bccdcc) to head (0e7b3f7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1184      +/-   ##
==========================================
+ Coverage   91.12%   91.21%   +0.08%     
==========================================
  Files          15       15              
  Lines        6117     6121       +4     
==========================================
+ Hits         5574     5583       +9     
+ Misses        543      538       -5     

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