fix: add descriptions for a bunch of subcommands

[catdevnull]

[catdevnull]

this also helps them show up in --help:

█▀▀█ █▀▀█ █▀▀ █▀▀▄ █▀▀ █▀▀█ █▀▀▄ █▀▀
█░░█ █░░█ █▀▀ █░░█ █░░ █░░█ █░░█ █▀▀
▀▀▀▀ █▀▀▀ ▀▀▀ ▀  ▀ ▀▀▀ ▀▀▀▀ ▀▀▀  ▀▀▀

Comandos:
  opencode mcp               manage MCP servers
  opencode [project]         start opencode tui                        [defecto]
  opencode run [message..]   run opencode with a message
  opencode generate          generate OpenAPI specifications
  opencode debug             debug tools and utilities
  opencode auth              manage credentials
  opencode agent             manage agents
  opencode upgrade [target]  upgrade opencode to the latest or a specific
                             version
  opencode serve             starts a headless opencode server
  opencode models            list all available models
  opencode stats             show usage statistics
  opencode github            manage GitHub agent