chore: exclude TODO.md from CRAN tarball via .Rbuildignore

Eliminates the 'Non-standard file/directory found at top level' NOTE
in R CMD check --as-cran.

Author: evandeilton

.Rbuildignore

@@ -13,4 +13,6 @@
 ^Meta$
 ^\.claude$
 ^\.Rhistory$
-.claude
+.claude
+^\.positai$
+^TODO\.md$