[codex] fix rule path validation for custom summary rules

[MickeyWzt]

Summary

Fixes #936.

This delays validation of the default rules directory until the CLI actually needs to walk a rules directory. Commands that provide a specific JSON rule with -s custom_rule.json no longer fail during Click option parsing just because the default rules directory has not been downloaded yet.

Root cause

The --rule option used click.Path(exists=True) with a default path. Click validates default values while parsing, so an absent default rules directory blocked commands even when the command used a specific rule file instead.

Changes

• Remove eager exists=True validation from the --rule Click option.
• Add an explicit missing-directory check before walking the rules directory.
• Add a regression test that sets the default rule path to a missing directory and verifies a custom summary rule file is still accepted.

Validation

• python -m pytest tests/test_cli.py -q passed.
• python -m pytest tests/test_cli.py tests/test_freshquark.py -q shows the new test passing, while the existing tests/test_freshquark.py tests fail on Windows path separator expectations (/ vs \\), unrelated to this change.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.89%. Comparing base (1df1d49) to head (2802f3c).

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

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #944   +/-   ##
=======================================
  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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[MickeyWzt]

I added a second regression test for the missing --rule directory branch and verified the focused CLI tests locally:

python -m pytest tests/test_cli.py -q
# 2 passed

One note on the Codecov patch failure: this looks like a workflow checkout issue for fork PRs rather than a missing test. The build workflow runs on pull_request_target, and the Actions log for the previous run shows actions/checkout fetching and checking out the base commit 1df1d49... from origin/master, while Codecov uploads that coverage report against the PR head commit. That means the uploaded coverage is generated from the base branch, not from this PR's changed files, so patch coverage reports the changed quark/cli.py lines as uncovered even though the new tests cover the added branch when run against the PR branch.

The latest push restores the source diff to the clean implementation and keeps the added regression tests focused on the behavior in this PR.