Drop the TODO.md section numbers from 26 files

Eighty-five comments cited section numbers -- 1.1, 2.2.6, 3.6 -- from a
numbering the file had already abandoned before the move to the tracker. They
label completed work, so the pointer was the only wrong part.

The citation is removed and the sentence kept, which is what issue #27
recommended: these are labels, not references, and a label carrying a
version-dependent pointer goes stale again at the next reorganisation. Where a
pointer earns its place it names what actually holds the content now --
UPGRADING.md for the confirmed defects, libakerror #15 for the target
namespacing, issue #7 for the mutation survivors.

README.md and akstdlib.h sent readers to TODO.md for 'what is still open';
they name the tracker.

Verified: cmake --build build && ctest --test-dir build, 19/19.

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

tests/test_path.c

@@ -1,12 +1,12 @@
 /*
- * aksl_realpath and aksl_realpath_alloc -- TODO.md section 1.5, now complete.
+ * aksl_realpath and aksl_realpath_alloc.
  *
  * The happy paths compare against realpath(3) itself rather than against a
  * hard-coded string, because $TMPDIR may itself be a symlink (/tmp -> /private/tmp
  * and friends) and the resolved answer is what the platform says it is.
  *
  * The failure cases now pass an *uninitialised* resolved_path on purpose. That
- * used to be the crash case (TODO.md 2.1.6): the wrapper's own error path
+ * used to be the crash case: the wrapper's own error path
  * formatted the buffer with %s while realpath(3) leaves its contents
  * unspecified on failure, so the library read uninitialised memory while
  * reporting an error. The message names only the input path now, and this test
@@ -122,7 +122,7 @@ static int test_rejects_null_arguments(void)
     AKSL_CHECK_STATUS_MSG_CONTAINS(aksl_realpath(NULL, resolved, sizeof(resolved)),
 				   AKERR_NULLPOINTER, "path=");
     /*
-     * TODO.md 2.1.6: this used to be unchecked, and realpath(path, NULL)
+     * this used to be unchecked, and realpath(path, NULL)
      * allocated a buffer that the wrapper then discarded and leaked.
      */
     AKSL_CHECK_STATUS_MSG_CONTAINS(aksl_realpath("/tmp", NULL, PATH_MAX),