fix: defer default rule path validation (#936)

[gitcommit90]

Summary

• Remove exists=True from the --rule option so Click does not reject the missing default path during parsing.
• Existing command logic still validates explicit JSON rule files and walks the rule directory.

Closes #936

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.89%. Comparing base (1df1d49) to head (ef9ea4c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #946   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files          81       81           
  Lines        7131     7131           
=======================================
  Hits         5626     5626           
  Misses       1505     1505           

Flag	Coverage Δ
unittests	78.89% <ø> (ø)

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[haeter525]

Hi @gitcommit90. thank you for opening the PR!

We're already working on this issue in #937, so I am going to close this PR to avoid duplication. Please feel free to share ideas or leave a comment there. Thanks.