Aggregator — `WorkItem[]` → `Ticket[]`

[abdotop]

Depends on: Issue 1, Issue 3, Issue 4, Issue 5, Issue 6

Wire extractKey, grouping, deriveStatus, and resolvePerson (Issues
1/3) together into one function that takes the combined output of all
three providers (Issues 4–6) and produces Ticket[]. See §4.

Acceptance criteria

• A Jira issue and its GitHub PR (same key) merge into one Ticket
  with two items
• Ticket.status is deduced correctly for at least the todo/open,
  in_progress/open, and in_progress/merged cases
• Ticket.assignees/reviewers are deduped Person[], not raw refs
• Unkeyed items still surface as their own single-item Ticket