deliverables: add Gartner preso folder + copy subdirs in build

Add deliverables/gartner/ (product-analytics-demo.html + 32 images).

Fix copyDeliverables() in scripts/copy-oneoffs.js: it only copied top-level
files, so nested deliverable folders (and their images) were dropped from the
build output. Now copies subdirectories recursively (skipping .DS_Store), so
the Gartner deck's relative image links resolve on GitHub Pages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ak--47