Migrate to TypeScript and port kotobae's quality gates and CI #2

Summary

Meta issue. Migrate motion-render from plain JavaScript (.mjs) to TypeScript, and port the TypeScript quality gates and GitHub Actions CI from umiyosh/kotobae. The work is split into the sub-issues below and tracked here.

Background

• The CLI is about 460 lines of JavaScript in skills/motion-render/scripts/ (cli.mjs, audio.mjs, work.mjs, render.mjs), started from bin/motion-render.
• There are no tests, no lint, and no CI. master is protected by a ruleset that requires one approval, but there is no status check to require yet.
• kotobae already has a working setup: ESLint (typescript-eslint) + Prettier + tsc + Vitest with an 80% coverage gate + a quality baseline gate (ESLint warnings and Knip), run on every pull request.

Sub-issues

Done in this order:

• Migrate the CLI scripts to TypeScript #3 Migrate the CLI scripts to TypeScript
• Port kotobae's quality gates and CI #4 Port kotobae's quality gates and CI
• Add unit tests, the coverage gate and a render smoke test #5 Add unit tests, the coverage gate and a render smoke test

The migration lands before the gates, because the ESLint, tsc and Knip settings ported from kotobae target TypeScript. The tests come last, because the 80% coverage gate needs both the typed code and the CI job.

Done when

All sub-issues are closed.