chore: 第二个误提交的 .mcpp/,以及上一次扫描为什么没看见它

examples/import-std/.mcpp/.xlings.json —— 与根下那个同源的实验残留(同样声明
xim:picolibc-riscv,本包同样没有任何地方用它)。

⚠️ 上一轮已经做过一次这个检查并报告「六个仓干净」。它漏掉这一个的原因是判据写成了
^(\.mcpp|target/…) —— **锚定在路径开头**,所以只看得见仓库根下的那一个。

⇒ 一个检查报告「干净」的时候,要先问它看得见多少。

Author: speak-agent

.gitignore

@@ -8,3 +8,7 @@ mcpp.lock
 # `.mcpp/.xlings.json` from an experiment was committed with the bare-metal
 # work and then travelled in the pull request.
 .mcpp/
+# ⚠️ One `.mcpp/` rule matches the directory at any depth, and it does not
+# remove a file that is ALREADY TRACKED. This repository had two: one at the
+# root and one under `examples/import-std/`. The first cleanup found only the
+# root one, because the scan it used anchored the path at the beginning.