feat(cli): add nemo storybook subcommand with --component and --search flags

[geoffjay]

Closes #50

Summary

• Added Commands::Storybook(StorybookArgs) to args.rs with --component and --search flags
• Maintained backward compatibility — all existing flat args (--app-config, --headless, etc.) are preserved
• Added launch_storybook() to main.rs that generates the storybook config and launches the GPUI app
• nemo storybook --component button deep-links to the button page on launch
• nemo storybook --search chart pre-fills the sidebar search filter

Part of epic #9.

[geoffjay]

This change is part of the following stack:

• fix(storybook): replace HCL code snippets with XML syntax in styling page #71
  • feat(cli): add nemo storybook subcommand with --component and --search flags #72 ◀

_{Change managed by git-spice.}