chore: 删掉误提交的 .mcpp/,并补上这个仓唯一缺的那条 .gitignore

.mcpp/.xlings.json 是裸机那次工作里的实验残留 —— 它声明 xim:picolibc-riscv,而本包
没有任何地方用它。

⚠️ 而它能进来的原因是可查的:这套生态里七个仓,**只有这一个**的 .gitignore 没有
.mcpp。补上。

⭐ 顺手把七个 PR 的改动文件全扫了一遍(对象/归档/日志/构建目录/临时文件),其余六个
干净。

Author: speak-agent

.gitignore

@@ -2,3 +2,9 @@ target/
 compile_commands.json
 mcpp.lock
 .cache/
+# ⚠️ THE ONE THIS REPOSITORY DID NOT HAVE, AND THE ONE THAT SLIPPED IN.
+# `.mcpp/` is where the build tool keeps a workspace's own state; every other
+# repository in this ecosystem ignores it and this one did not, so a
+# `.mcpp/.xlings.json` from an experiment was committed with the bare-metal
+# work and then travelled in the pull request.
+.mcpp/