fix: use C syntax in tdd skill examples

auto trailing return types are C++ only. Split header and\nimplementation examples into C and C++ variants.

.pi/skills/tdd/SKILL.md

@@ -38,10 +38,21 @@ before the implementation was missing.
 
 Create `<src>/<module>.h` with the public prototype(s).
 
+C:
+
 ```c
 #pragma once
 #include <stddef.h>
 
+int fn_name(int arg);
+```
+
+C++:
+
+```cpp
+#pragma once
+#include <cstddef>
+
 auto fn_name(int arg) -> int;
 ```
 
@@ -122,9 +133,21 @@ list(APPEND TEST_TARGETS test_<module>)
 
 Create `<src>/<module>.c` with a dummy return:
 
+C:
+
 ```c
 #include "<src>/<module>.h"
 
+int fn_name(int arg) {
+    return 0;
+}
+```
+
+C++:
+
+```cpp
+#include "<src>/<module>.h"
+
 auto fn_name(int arg) -> int {
     return 0;
 }