refactor: restructure the whole plugin following a better separation of concers

lua/jj/core/runner.lua

@@ -0,0 +1,33 @@
+--- @class jj.core.runner
+local M = {}
+
+--- Execute a system command and return output with error handling
+--- @param cmd string The command to execute
+--- @param error_prefix string|nil Optional error message prefix
+--- @param input string|nil Optional input to pass to stdin
+--- @param silent boolean|nil Optional to silent the notification
+--- @return string|nil output The command output, or nil if failed
+--- @return boolean success Whether the command succeeded
+function M.execute_command(cmd, error_prefix, input, silent)
+	local output = vim.fn.system(cmd, input)
+	local success = vim.v.shell_error == 0
+
+	if not success then
+		local error_message
+		if error_prefix then
+			error_message = string.format("%s: %s", error_prefix, output)
+		else
+			error_message = output
+		end
+		if not silent then
+			vim.notify(error_message, vim.log.levels.ERROR, { title = "JJ" })
+		end
+
+		return nil, false
+	end
+
+	return output, success
+end
+
+return M
+