fix: validate default rules path only when used

[qinfenyinluo]

Summary

• Allow --rule to be parsed even when the default rules directory has not been downloaded yet.
• Report a clear warning only when Quark actually needs to load rules from the configured rules path.
• Add CLI regression tests for using a custom single rule file while the default rules path is missing.

Root cause

--rule used click.Path(exists=True) together with the default rules directory. Click validated that default during argument parsing, so commands that supplied their own rule file via --summary custom_rule.json could fail before Quark reached the code path that uses the custom rule.

Validation

• python -m pytest tests\test_cli.py -q -> 2 passed
• python -m black --check tests\test_cli.py -> passed
• git diff --cached --check -> passed before commit

I also tried python -m pytest tests\test_cli.py tests\test_freshquark.py -q. The new CLI tests passed, but the existing tests/test_freshquark.py assertions fail on Windows because expected paths use \ while the mocked command builds paths with /; this appears unrelated to this change.

Closes #936

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.89%. Comparing base (cf35a66) to head (3a2bf3d).

Files with missing lines	Patch %	Lines
quark/cli.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #945   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files          81       81           
  Lines        7131     7131           
=======================================
  Hits         5626     5626           
  Misses       1505     1505           

Flag	Coverage Δ
unittests	78.89% <0.00%> (ø)

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[qinfenyinluo]

Closing this as duplicate after noticing that #937 already covers the same issue. Sorry for the extra noise, and thanks for maintaining Quark.