Update .gitignore to include a4d prompts in source control

[mvogelgesang]

With the ability to create reusable prompts for Agentforce for Developers (A4D) the default .gitignore should be updated to allow for the new files. The post announcing the new capabilities: https://trailhead.salesforce.com/trailblazer-community/feed/0D5KX00000SWKOY0A5

 # Salesforce cache
 .sf/
-.sfdx/
+/.sfdx/*
+!/.sfdx/a4d/

[cristiand391]

closing this since A4D is now Agentforce Vibes and it saves rules to rules in a different dir