fix(autoupdate): robust wait-for-checks logic before merge

[siarheidudko]

Summary

• Fix false-positive exit from the "wait for checks" loop: grep -c '\S' matched the "no checks reported on the '...' branch" message before any real check runs were registered, causing the loop to break prematurely
• Replace with grep -cE 'https?://' — only actual check run entries contain a URL
• Add || true after gh pr checks --watch to prevent step abort on non-zero exit
• Add explicit failure gate before merge
• Increase polling retries from 10 to 20

Test plan

• Merge this PR to main
• Verify next autoupdate run auto-merges the PR after checks pass

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