Implement slash command system #11773

// Command registry
interface Command {
  name: string
  description: string
  handler: (args: string[], context: CommandContext) => Promise<void>
}

interface CommandContext {
  dispatch: Dispatch<Action>
  session: SessionState
  setUIMode: (mode: UIMode) => void
}

const commands: Command[] = [
  {
    name: 'help',
    description: 'Show available commands',
    handler: async (args, ctx) => {
      ctx.dispatch({ type: 'SHOW_HELP' })
    }
  },
  {
    name: 'agent',
    description: 'Switch agent',
    handler: async (args, ctx) => {
      if (args[0]) {
        await switchAgent(args[0])
      } else {
        ctx.setUIMode('select')
        ctx.dispatch({ type: 'SHOW_AGENT_SELECT' })
      }
    }
  },
  // ...
]

// Command execution
function executeCommand(input: string, context: CommandContext) {
  const [cmd, ...args] = input.slice(1).split(' ')
  const command = commands.find(c => c.name === cmd)
  if (command) {
    return command.handler(args, context)
  }
  throw new Error(`Unknown command: ${cmd}`)
}