chore: keep internal planning notes out of the public repo

BLOG-BACKLOG.md and REBRAND-PLAN.md were tracked in a public repo. Both are
internal planning material — an unpublished content backlog and a rebrand plan —
so they belong next to promotion/ and legal/, which are already ignored for the
same reason.

Their history was purged in the same pass, so past commits no longer carry them
either. Local copies are kept; only the tracking is gone.

Author: Mikhus

.gitignore

@@ -43,3 +43,8 @@ legal/
 # Promotion working material — drafts, playbook and syndication exports.
 # Local only: never commit this, the repo is public.
 promotion/
+
+# Internal planning notes. Both were tracked and were purged from history on
+# 2026-07-31; local only from now on, for the same reason as promotion/ above.
+BLOG-BACKLOG.md
+REBRAND-PLAN.md