fix(kanban): dashboard link reports the gate; docs; trim to two invariant tests

The dashboard's POST /links is the fourth writer of link_tasks (CLI, tool,
dashboard, plus the graph builder); return the same ``gated`` flag so every
surface that can create the deadlock can see it. Document the
``dependency_wait`` payload the link path emits and the delegation rule the
reporter derived (never link a support card under the card it unblocks).

Drops the CLI output test (a change-detector on prose); the two DB-level
invariants (event emitted on demotion / none for a done parent) stay.

plugins/kanban/dashboard/plugin_api.py

@@ -765,8 +765,8 @@ class LinkBody(BaseModel):
 @router.post("/links")
 def add_link(payload: LinkBody, board: Optional[str] = Query(None)):
     with _board_conn(board) as (board, conn), _value_error_400():
-        kanban_db.link_tasks(conn, payload.parent_id, payload.child_id)
-        return {"ok": True}
+        gated = kanban_db.link_tasks(conn, payload.parent_id, payload.child_id)
+        return {"ok": True, "gated": gated}
 
 
 @router.delete("/links")