chore: stop tracking internal design drafts and plans

docs/design/ and docs/plans/ are working documents, not repository
content; keep them local and ignore the directories so they cannot be
swept into a commit again. User-facing docs (RELEASE.md,
github-action.md, ai-integration.md) stay tracked.

Author: DropFan

.gitignore

@@ -33,3 +33,7 @@ criterion/
 
 # Documentation build
 /target/doc/
+
+# Internal design drafts and plans (kept local, not tracked)
+docs/design/
+docs/plans/