fix(release): make installed package artifacts repackable

[drewstone]

Summary

• replace package prepare builds with prepublishOnly across all eight publishable packages
• add one repository-level artifact check that discovers every publishable package, installs clean tarballs with scripts enabled, imports every public export, and repacks every installed package
• run the artifact check in CI and before the Changesets release action

Root cause

Published manifests retained prepare: pnpm run build. npm runs prepare while installing and repacking, but published tarballs omit the source and development dependencies needed by that build. Moving the build to prepublishOnly keeps publish-time builds without making installed artifacts rebuild themselves.

Validation

• pnpm install --frozen-lockfile
• pnpm build
• pnpm check-types
• pnpm test (548 passed)
• pnpm check:package-artifacts (8 isolated packages, 18 exports, 36 export targets)
• pnpm changeset status
• temporary pnpm changeset:version release proof followed by build and the full package-artifact check
• pnpm --filter @tangle-network/agent-interface check-types
• pnpm --filter @tangle-network/agent-interface test (115 passed after removing the invalid locator change)
• repository-wide rg content-store (no matches)
• git diff origin/main...HEAD --check
• git merge-tree --write-tree origin/main HEAD

No package was published and no branch was merged.