Add isChildOfEpic() function that checks if an issue is a child of
the configured epic by verifying the issue has the epic in its
dependents with dependency_type="blocks".
When EpicID is configured in witness config, only issues that block
that epic will be considered for auto-spawning. Issues that cannot
be verified are safely skipped.
Closes gt-zhm5.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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