Replace commander with yargs and restructure CLI

[threeal]

This pull request resolves #1119 by:

• Replaces commander with yargs for argument parsing, adding @types/yargs as a dev dependency
• Restructures the CLI from a single src/cli.ts into src/cli/index.ts (entry point) and src/cli/commands/generate.ts (the generate command module)
• Each command is exported as a createXxxCommand({ stdout }) factory, abstracting I/O for testability
• Replaces the process-level cli.test.ts integration test with a unit test that wires up a real yargs instance with .exitProcess(false) and a Writable stream
• Updates package.json bin path to dist/cli/index.js and refreshes CLAUDE.md / README.md to reflect the new structure