chore: remove internal marketing campaign doc from repo

Move docs/marketing-campaign.md to ai-plans/ (gitignored) for
internal use only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Alan Shum

.gitignore

@@ -1 +1,2 @@
 target/
+ai-plans/