feat(bot): !play id <id> 与其他命令语法保持一致

按 id 精确播放原本要写 `!play id:<id>`，冒号在一堆 `!<命令> <子命令> <参数>`
的命令里显得很突兀。现在空格写法 `!play id <id>` 也可以，`!add` / `!playnext`
共用同一个解析器，一起生效。

`id:<id>` 继续支持，不做废弃：用户的聊天记录、旧文档和 !search 输出里都是
这个写法。

冒号是个明确的标记，所以 `id:<任意内容>` 一律当 id。空格不是——「ID 4」和
「ID Bruno」都是真实存在的歌名，而且 `id <链接>` 原本会落到 URL 分支正常解析。
所以空格写法只认「长得像 id」的 token（纯数字 / BV 号 / 11 位以上的 id 字符），
其余照旧继续走 URL 识别，最后落到普通搜索，不会把搜索词误当成 id。

同步更新 !search 输出的提示、三条 Usage、!help 和 README。

Closes #139

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/bot/song-ref.test.ts

@@ -21,6 +21,59 @@ describe("parseSongRef (#90 exact-song selection)", () => {
     expect(parseSongRef("id:185868,")).toEqual({ id: "185868", platform: null });
   });
 
+  // Issue #139: `!play id <id>` matches the "<command> <subcommand> <arg>"
+  // shape of every other command. The colon form stays supported — users have
+  // it in their chat scrollback and in older docs.
+  it("parses the space-separated id form", () => {
+    expect(parseSongRef("id 185868")).toEqual({ id: "185868", platform: null });
+    expect(parseSongRef("ID 004Z8Ihr0JIu5s")).toEqual({ id: "004Z8Ihr0JIu5s", platform: null });
+    expect(parseSongRef("id   185868")).toEqual({ id: "185868", platform: null });
+    expect(parseSongRef("id 185868.")).toEqual({ id: "185868", platform: null });
+  });
+
+  it("does not mistake a word merely starting with 'id' for an id reference", () => {
+    expect(parseSongRef("idol")).toBeNull();
+    expect(parseSongRef("identity 185868")).toBeNull();
+    expect(parseSongRef("id")).toBeNull();
+    expect(parseSongRef("id:")).toBeNull();
+    // Two remaining tokens are a search phrase, not an id.
+    expect(parseSongRef("id die for you")).toBeNull();
+  });
+
+  // Without a colon, "id" is just a word — "ID 4" and "ID Bruno" are real track
+  // titles. The space form therefore only claims tokens that could actually be
+  // an id; everything else stays a search term.
+  it("only treats the space form as an id when the token looks like one", () => {
+    expect(parseSongRef("id Bruno")).toBeNull();
+    expect(parseSongRef("id Marshmello")).toBeNull();
+    expect(parseSongRef("id 4ever")).toBeNull();
+    // …while every real id shape is still accepted.
+    expect(parseSongRef("id 4")).toEqual({ id: "4", platform: null });          // numeric
+    expect(parseSongRef("id BV1yxHQeYEuE")).toEqual({ id: "BV1yxHQeYEuE", platform: null });
+    expect(parseSongRef("id 004Z8Ihr0JIu5s")).toEqual({ id: "004Z8Ihr0JIu5s", platform: null }); // QQ mid
+    expect(parseSongRef("id a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6")).toEqual({
+      id: "a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6",
+      platform: null,
+    }); // Jellyfin GUID / Kugou hash
+  });
+
+  it("keeps the colon form unrestricted, so a short or odd id still works", () => {
+    expect(parseSongRef("id:Bruno")).toEqual({ id: "Bruno", platform: null });
+    expect(parseSongRef("id: 4ever")).toEqual({ id: "4ever", platform: null });
+  });
+
+  it("does not let the space form swallow a pasted URL", () => {
+    // `id <url>` used to fall through to the URL branches; it still must.
+    expect(parseSongRef("id https://music.163.com/song?id=185868")).toEqual({
+      id: "185868",
+      platform: "netease",
+    });
+    expect(parseSongRef("id https://y.qq.com/n/ryqq/songDetail/004Z8Ihr0JIu5s")).toEqual({
+      id: "004Z8Ihr0JIu5s",
+      platform: "qq",
+    });
+  });
+
   it("does NOT treat NetEase collection (playlist/album/artist) URLs as a song id", () => {
     // These reuse ?id= but are not songs — they should fall through to search,
     // not misresolve to getSongDetail(collectionId) and error "no song".