Add Shopstr final reflection blog

[ayushshrivastv]

Summary

• Add Ayush Srivastava final Shopstr/Summer of Bitcoin reflection post.
• Format the pasted draft with required Jekyll front matter, categories, section headings, Nostr featured image, and work links.
• Fold the Shopstr invitation into the conclusion.

Validation

• git diff --check
• bundle exec jekyll build

Note: the Jekyll build passes, while still reporting existing Sass @import deprecation warnings and existing duplicate category archive destination warnings.