was: `print_event` did the format + println inline; tests had a
parallel `captured` helper that re-implemented the dispatch and only
covered 4 of 14 event types. brittle — a new arm in `print_event`
silently went uncovered.
now: pure `format_event(ev: &Value) -> String` builds the line;
`print_event` is a thin wrapper that adds the trailing newline.
tests assert on `format_event` output directly so every new arm
gets test coverage by construction.
bonus: added 3 more test cases (assignees add/remove, pull_push
commit count + force-push, commit_ref sha truncation) since the
helper extraction made them cheap. 9 tests total, all green.
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