feat(scaffold-core): npm publish pipeline for @stackbilt/scaffold-core

[stackbilt-admin]

Part of #212. Blocked on: #213 #214 #215 #216 #217.

Scope

Set up the publish pipeline for @stackbilt/scaffold-core in the charter monorepo, following the existing pattern from @stackbilt/adf, @stackbilt/blast, etc.

Checklist

• packages/scaffold-core/package.json with correct name, exports map, files field
• tsconfig.json extending root, ESM output
• vitest.config.ts
• GitHub Actions CI (build + test on PR)
• OIDC trusted-publisher npm publish (same as other packages + @stackbilt/build)
• Initial version 0.1.0 — API marked experimental until all three consumers migrated
• Update root pnpm-workspace.yaml to include packages/scaffold-core

[stackbilt-admin]

Work shipped in @stackbilt/scaffold-core@1.1.0 (charter#212, PRs #219-222). Closing stale tracking issue.