Introduces a yargs-based CLI entry point (src/cli.ts) distributed as the
`game-ci` command. The CLI reuses existing unity-builder modules — Input,
BuildParameters, Orchestrator, Docker, MacBuilder — so the same build
engine powers both the GitHub Action and the standalone CLI.
Commands: build, activate, orchestrate, cache (list/restore/clear),
status, version.
Closes#812
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
