Note directory-negation limit in codebuffignore docs

Author: nordicnode

sdk/README.md

@@ -183,7 +183,9 @@ terminal-command or internal-edit restrictions.
 Ignore-rule blocks on `read_files` return `[BLOCKED]` followed by a trailing
 reason. `.codebuffignore` — a project-level ignore file with `.gitignore`
 syntax, checked alongside `.gitignore` — is the escape hatch: adjust or
-negate the matching rule there to allow tool reads of a specific file.
+negate the matching rule there to allow tool reads of a specific file (a
+file-level negation cannot re-include a path whose parent directory is itself
+excluded).
 
 Custom `overrideTools.read_files` implementations must preserve project
 gitignore behavior for env templates. The built-in CLI, Desktop, Web, and Cloud