dedupeByIssueId: distinguish multi-PR epics from backport cherry-picks #154

Raised in PR #138 review (#138 (comment)).

Backport cherry-picks and multi-PR epics collapse on the same issue-id key. Deleting a backport copy is safe; deleting one part of an epic loses content, and the surviving draft still advertises source_prs as if it covered all of them.

Proposed direction: only auto-collapse when the group looks like a backport (e.g. titles match modulo the trailing (#NNNN) suffix / cherry-pick marker); otherwise keep all members and flag the group for human review. Needs a design decision on the discriminator.