Party’s return to minority government after a year of upheaval will test his renowned negotiating skillsJohn Swinney declares ‘new chapter’ as he becomes SNP leaderFew people understand the internal dynamics and historical loyalties of the Scottish National party better than John Swinney, who joined the party at 15 and quickly rose to become national secretary by his early 20s.His unfussy election as leader says much about the esteem in which he is held by colleagues across factions and ages – while at 60 Swinney is a generation older than his predecessor, Humza Yousaf, his confirmation was instantly welcomed by Amy