#2799 gatorwalk-factory: mark a duplicate and move its work to the primary

Part of the tracker-seam decision (https://claude.ai/artifact/TDidgyVjxKXU7JsNH1baNy). Needs #2796 and #2798.

Seth's calls: an agent marks a duplicate with approval, or a person marks it directly. One issue is the primary, and in-flight work always moves to the selected primary.

Change

• A tracker method mark_duplicate --input issue=<dup> --input primary=<p> for a person. A person running it is the approval.
• For an agent, a lifecycle pattern using the engine's existing gates. swamp-extensions.yaml triage gets a duplicate exit that records a duplicate-of product behind a human-approval gate, and the stage's tracker block names mark_duplicate as the effect.
• Effect:
  1. Write duplicate_of in the backend.
  2. Close the duplicate where the backend does not (the Lab does not).
  3. Move the work:
     • the duplicate has an active work item and the primary none: print retarget onto the primary, and move the ticket index;
     • both have one: the primary's continues, and the duplicate's is abandoned through its global exit with a pointer to the primary;
     • otherwise nothing moves.
• Marked outside gatorwalk (for example in the Lab): publish refuses to project to a duplicate and prints the same move. claim on a duplicate points at the primary.

Done when

• The built-in tracker and the Lab fake cover all three work-item cases, the approval path, and a duplicate marked outside gatorwalk.
• The driving skill documents both paths.
• The coordinator's held linked-issues draft (issue-lifecycle's link_issue/mark_duplicate from #2765) is superseded, and this issue says so.