The inline replaceSpecialAssigneeNames closures in AssigneeIds and
AssigneeLogins were duplicated. Extract them into an exported
SpecialAssigneeReplacer type that consolidates MeReplacer and
CopilotReplacer into a single ReplaceSlice call, parameterised by
actorAssignees and copilotUseLogin.
Adopt the new type in the issue create flow as well, replacing the
manual MeReplacer + conditional CopilotReplacer sequence.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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