gitignore: protect rough_drafts/*.txt from accidental commits

The user's drafts in rough_drafts/ are real in-progress academic
work. Project discipline (per memory) is to keep them untracked,
but a single accidental `git add -A` could leak unpublished
scholarship into git history. Adding the pattern to .gitignore
makes the protection structural rather than discipline-based.

Scoped to .txt only (the docx->txt converted form the project
actually consumes); does not affect sample.md, .gitkeep, or any
other extensions that might land in rough_drafts/ later.

.gitignore

@@ -14,3 +14,6 @@ logs/*
 .env
 .env.*
 !.env.example
+
+# rough drafts: real in-progress academic work, do not commit
+rough_drafts/*.txt