add agent picker

[Robitx]

telescope example thanks to @qaptoR

      local pickers = require 'telescope.pickers'
      local finders = require 'telescope.finders'
      local actions = require 'telescope.actions'
      local action_state = require 'telescope.actions.state'
      local conf = require('telescope.config').values

      local models = function(opts)
        local buf = vim.api.nvim_get_current_buf()
        local file_name = vim.api.nvim_buf_get_name(buf)
        local is_chat = require('gp').not_chat(buf, file_name) == nil

        opts = opts or {}
        pickers
          .new(opts, {
            prompt_title = 'Models',
            finder = finders.new_table {
              results = is_chat and require('gp')._chat_agents or require('gp')._command_agents,
            },
            sorter = conf.generic_sorter(opts),
            attach_mappings = function(prompt_bufnr)
              actions.select_default:replace(function()
                local selection = action_state.get_selected_entry()
                actions.close(prompt_bufnr)
                require('gp').cmd.Agent { args = selection[1] }
              end)
              return true
            end,
          })
          :find()
      end

      vim.keymap.set('n', '<C-g>z', function()
        models(require('telescope.themes').get_dropdown {
          winblend = 10,
          previewer = false,
        })
      end, {
        noremap = true,
        silent = false,
        nowait = true,
        desc = 'GPT prompt Choose Agent',
      })

Originally posted by @qaptoR in #158