Remove the no-op --collection option from get and get-thumb (closes #241)

Both commands listed `--collection <id>` as accepted and ignored. It now
is refused as an unknown option, and the README no longer mentions it.

Model: opus-5-5

test/cli/commands.test.ts

@@ -569,14 +569,14 @@ describe("files", () => {
 });
 
 describe("get and get-thumb", () => {
-    it("get finds a file in any album without --collection", async () => {
+    it("get finds a file in any album", async () => {
         const out = join(root, "diagram.png");
         expect(await getCommand(context(), "200", { out })).toBe(0);
         expect(readFileSync(out)).toEqual(Buffer.alloc(7, 200));
         expect(stderr.text).toBe(`7 bytes -> ${out}\n`);
     });
 
-    it("get-thumb finds a file in any album without --collection", async () => {
+    it("get-thumb finds a file in any album", async () => {
         const out = join(root, "thumb.jpg");
         expect(await getThumbCommand(context(), "200", { out })).toBe(0);
         expect(readFileSync(out)).toEqual(Buffer.alloc(3, 200));