feat(cli): Replace btw info subcommands with btw commands

[gadenbuie]

• btw info platform → btw system-info
• btw info packages → btw check-installed and btw installed-packages

❯ btw --help
Usage: btw [OPTIONS] <COMMAND>

Describe R objects, documentation, and workspace state in LLM-friendly text.
Wraps btw package tools for docs, pkg, info, and cran operations.

Commands:
  docs                Access R documentation
  pkg                 Work with an R package under development
  info                [Deprecated] Inspect the R session and environment
  system-info         Show platform and R session info
  check-installed     Check if packages are installed
  installed-packages  Show installed package versions
  cran                Query CRAN package metadata
  skills              Manage btw skills
  help                Show btw CLI usage guide for AI agents
  app                 Run btw_app() in the current directory

Options:
  --version / --no-version  Print btw version and exit. [default: false]
                            Enable with `--version`.

For help with a specific command, run: `btw <command> --help`.