Surface the CEO self-review guard as a proper 422

The database layer already refuses to advance a top-level assignee's
task to in_review without requires_user_review=1 — the right
invariant. But the HTTP route caught nothing specific and returned a
bare 500, so the CEO prompt had to carry a paragraph of "the system
will demote any such task back to todo, but the cycle wastes a
heartbeat slot" prose to keep the model away from the error path.

Translate the guard to a 422 NO_REVIEWER_ON_TOP_LEVEL_TASK response
with an actionable hint and rewrite the prompt section to describe
the contract instead of begging the model to avoid the failure.
Also drop the residual skill-looping paragraph now that hires get
their default kit on approval.

src/main/api-server/routes-tasks.ts

@@ -278,6 +278,17 @@ export function registerTaskRoutes(server: AgentApiServer) {
         server.json(res, 404, { error: error.message });
         return;
       }
+      // Top-level assignee self-review guard: translate to a 422 so the
+      // agent sees an actionable business-logic rejection rather than a
+      // generic server failure.
+      if (error instanceof Error && error.message.includes("cannot enter in_review")) {
+        server.json(res, 422, {
+          error: error.message,
+          code: "NO_REVIEWER_ON_TOP_LEVEL_TASK",
+          hint: "Either reassign the task to a report, mark it done directly if the work is complete, or set requires_user_review=1 if the Board should review.",
+        });
+        return;
+      }
       server.json(res, 500, { error: error instanceof Error ? error.message : "Task update failed" });
     }
   });