fix(agents): system_prompt in tools.groups can be a function

[ravitemer]

Description

system_prompt of tools.groups being a function makes it dynamic and the following possible.

        groups = {
            mcp = {
                description = "MCP Servers Tool",
                system_prompt = function(_)
                    -- get the running hub instance
                    local hub = require("mcphub").get_hub_instance()
                    return hub:get_active_servers_prompt() -- generates prompt from currently running mcp servers
                end,
                tools = {"use_mcp_tool","access_mcp_resource"},
            },
        },

Related Issue(s)

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Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I've updated the README and/or relevant docs pages
• I've run make docs to update the vimdoc pages