fix: add CLI version flag

[120850853]

Summary

• register Commander's top-level version option from package.json
• add CLI integration coverage for claudemux --version

Reproduction

Published @wastedcode/claudemux@0.2.1 handles --help, but --version is rejected as an unknown option:

--help status=0 signal=null
--version status=1 signal=null stderr="error: unknown option '--version'\n"

Verification

• npm run build
• npx vitest run test/cli/cli.test.ts
• npm run typecheck
• npm run check
• npm pack + clean install tarball smoke for claudemux --help and claudemux --version