Use Clipanion for CLI implementation

[threeal]

Replaces Commander with Clipanion for the CLI implementation, and restructures the CLI source files to follow the barrel export convention established in #1114.

Changes

• Replaces commander dependency with clipanion
• Restructures CLI from a flat src/cli.ts into a module:
  • src/cli/index.ts — bin entrypoint; registers commands and calls cli.runExit()
  • src/cli/commands/fibonacci.ts — FibonacciCommand class using this.context.stdout instead of process.stdout, with full static usage (description, details, examples)
• Tests are scoped to the command level only (src/cli/commands/fibonacci.test.ts), using a PassThrough stream as the context stdout for clean isolation
• Updates package.json bin field from dist/cli.js to dist/cli/index.js

Closes #1117

[threeal]

Apparently, Clipanion has a somewhat unusual help output. It looks different from the help output generated by Commander.

[threeal]

Surpassed by #1120.