Completing a parent silently skips children that cannot be looked up #78

Disclaimer: AI-generated text below the line, generated by Claude Opus 5.5, from the prompt(s):

Make a github issue from it [the review finding that an unresolvable child task is skipped without a warning when its parent is completed]

Since #76, completing a task also completes its pending child tasks, with a warning for each. Children are resolved from the parent's RELATED-TO;RELTYPE=CHILD lines via _relships_by_type, which looks each UID up only in the parent's own calendar (cache.get_object(obj.parent, uid)) and silently skips a NotFoundError.

So when a child lives in a different calendar, or the link is dangling, the parent is marked completed while that child stays pending, and the user gets no message about it.

Possible fixes:

• warn through err_callback when a CHILD UID cannot be resolved, at least on the completion path;
• look the UID up in the other selected calendars before giving up;
• or at least document the limitation in USER_GUIDE.md, next to the existing note that a child linked only by RELTYPE=PARENT is not found.

Found by a code review of PR 76.