fix: ignore fully skipped startup failures

[panva]

GitHub Actions can report a check suite as STARTUP_FAILURE even when every check run in that suite was skipped. This caused checkGitHubCI() to mark the PR as unlandable without reporting a specific failed job or reason code.

Ignore this suite-level result when every check run was fetched and every run completed with SKIPPED. Other startup failures continue to block landing and now fall back to a suite-level diagnostic when no failed check run is available.

Query the check-run totalCount to ensure truncated results are not mistaken for a fully skipped suite.

Observed in nodejs/node#65538 (comment)

https://github.com/nodejs/node/actions/runs/32985142178/attempts/1
https://github.com/nodejs/node/actions/runs/32984879569/attempts/1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.86%. Comparing base (fdcfe10) to head (41303bb).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1170      +/-   ##
==========================================
+ Coverage   57.81%   57.86%   +0.04%     
==========================================
  Files          53       53              
  Lines       10397    10409      +12     
==========================================
+ Hits         6011     6023      +12     
  Misses       4386     4386              

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