Model packs (model-packs)

feat(scripts): add model-pack build and release tooling

Add initial tooling and data to build and publish Happier voice model packs.

- add scripts/build-model-packs.mjs to assemble packs, prepare release artifacts, and compute checksums
- add scripts/licensePackaging.mjs and test for installing license notices into packs
- add packs/packs.json with initial pack definitions
- add GitHub Actions workflow (.github/workflows/release-model-packs.yml) to publish release assets (with clobber)
- add README, package.json (build script), .gitignore, and license texts (Apache-2.0, GPL-3.0)

Usage: npm run build:model-packs -- --repo <owner/repo> --tag <tag>

This commit bootstraps repository structure for producing dist/packs and dist/release artifacts consumed by the Happier apps.