tree: a burst of two same-name same-role calls names that Bot twice in the lead's role text #840

Two create_child_bot calls that are in flight at the same time with the same name and the same role become one lead with one child — and that lead's role text says the name twice: Lead of the FlyBot group: coordinates FlyBot Programmer, FlyBot Programmer.

Why (main be48393b)

• A burst takes one seat per call and marks the seats that share a key as a group: plugins/tree/src/allocate.js:125-136. The group's names are collected straight from the seats — seats.filter(o => o.key === key).map(o => o.name) (:135) — so two calls named the same contribute the name twice.
• Those names are handed to the one shared lead as memberNames (:204 → createLead), and createLead writes them into the lead's role text verbatim: Lead of the ${label} group: coordinates ${memberNames.join(', ')}. (plugins/tree/src/organize.js:173).
• The child itself is not duplicated: the second call reaches planChild and finds the Bot the first one made, so it reuses it (allocate.js:98, :213-214). Result: one child, a role text naming it twice.

This is not #819. #819 stopped a call from taking a seat when it would reuse an already existing Bot (:120-124). Here both calls do create work and both seats are real; they only collapse into one Bot later, in the reuse branch.

Acceptance criteria

1. Two concurrent create_child_bot calls from one caller with the same name and same role produce: one member Bot, one lead, and a lead role text in which that name appears once (coordinates FlyBot Programmer.), with the group label unchanged.
2. A burst whose members have different names still lists every name once, in seat order — deduping must not drop a member that was not reused.
3. The four burst cases from Tree: a reachable Archived group in the sidebar, and one group for a concurrent burst #810 (core-free plugins/tree suite: two calls become one group, different keys do not, a call that starts after the previous returned stays flat, area:-first keys are excluded) stay green.
4. Negative control: with the dedupe removed, the new case fails on the duplicated name only — the created-Bot count and the child count in the same test stay correct, so the test cannot pass for the wrong reason.

Kill conditions — do not:

• dedupe by role text or by a mangled name (use the same normalisation the reuse check uses, norm(name));
• change how seats are counted, or drop a seat before the reuse decision (planChild) has run;
• make the two calls create two Bots instead of one (the host refuses duplicate names by design, A Bot cannot be moved to another parent, so a team can only be regrouped by hand #764).

Refs #819 (seat taken for a reuse), #831 (the fix that landed for it), #764 (a Bot's name is unique across the roster).