Let the coordinator create bots and put them in workspaces

The coordinator had no tool to add a worker, so a request to create one
fell back to update_workspace and wrote a bot that did not exist into the
workspace context. create_bot now adds a worker with its name, role and
instructions and joins it to a workspace, and add_bot_to_workspace puts an
existing worker in one. Both are coordinator-only and chat-replies-only,
and createBot still enforces the crew-manager check.

The coordinator's prompts now say how a bot gets created, and that one
which does not exist is never written into workspace context.

pick-free-hue moves to utils so the tool can pick a hue without importing
from layouts.

src/utils/pick-free-hue.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, test } from "bun:test";
+import pickFreeHue from "./pick-free-hue";
+
+describe("pickFreeHue", () => {
+    test("takes the first hue nobody has", () => {
+        expect(pickFreeHue({ taken: [] })).toBe("ember");
+        expect(pickFreeHue({ taken: ["ember", null, "amber"] })).toBe("moss");
+    });
+
+    test("falls back to the least used hue", () => {
+        const all = ["ember", "amber", "moss", "teal", "cobalt", "violet", "rose"];
+        expect(pickFreeHue({ taken: [...all, ...all.filter((h) => h != "teal")] })).toBe("teal");
+    });
+});