Add Showcase package publishing

[celesteanglm]

Summary

• Add the Showcase project package layout under showcase/<project-slug>/ with the Paid Substack reference package.
• Add the PR template, manifest validator, and dispatch workflow that notifies the EconomyOS docs repo after Showcase changes merge.
• Document project-specific skills inside the showcase package, shared top-level skills, and hidden: true for packages that should merge without publishing a public card yet.

Tests

• node scripts/validate-showcase.mjs
• tmp_home=$(mktemp -d) && scripts/install-local-skills.sh --mode copy --target both --home "$tmp_home" && rm -rf "$tmp_home"

Paired docs work

• EconomyOS docs Showcase sync/contribution PR: https://github.com/Virtual-Protocol/whitepaper-economyOS/pull/51