Merge pull request 'Document libc wrapper contract' (#30) from 9 into main

Reviewed-on: #30

AGENTS.md

@@ -76,6 +76,10 @@ return convention and the `PREPARE_ERROR` / `FAIL_*` / `SUCCEED_RETURN` pattern.
 Four conventions hold across the whole library, and a new wrapper that breaks one
 of them is wrong even if it compiles and passes:
 
+- **Preserve the libc contract unless there is a compelling, documented reason
+  not to.** libc behaviour is the standard to meet. This library changes only
+  the error transport (to `akerror`) and, where libc returns a value, the result
+  shape (through a caller-provided destination pointer).
 - **A NULL out-param is a caller error**, not "don't care".
 - **Finding nothing is success** -- searching functions write NULL or zero and
   return NULL.
@@ -93,6 +97,11 @@ The build is `-Wall -Wextra` and CI adds `-Werror`. `-Wpedantic` is deliberately
 off: libakerror's `FAIL_*` macros trip "ISO C99 requires at least one argument
 for the ..." on their own expansion, not on anything at the call site.
 
+**Do not wrap a libc function that cannot fail and provides no failure or
+operation-status result.** There is no `akerror` context to carry. A value such
+as `umask()`'s previous mask is not an operation-status result, so `umask()` is
+not a wrapper candidate.
+
 ## Testing Guidelines
 
 Add a new test by creating `tests/test_mything.c` and adding `mything` to the