Allow GC artifact to be set prior to calling initial conversation step.

[momuno]

GC Configs

• Updates doc agent's gc naming to add clarity
• add filenames to gc attachment check config
• add filenames and current outline to gc outline feedback config

Doc agents' GC helper file updated:

• separate out GC constructor from conversation step --> use constructor to set initial state in storage
• add getter for artifact --> only reads from storage
• add setter for artifact --> writes to storage AND resets the GC instance with this new info
• step_conversation shrinks... only needs last_user_message now in parameters.

Doc agent

• updates logic per changes in GC helper file to initiate GC, update artifact, and run conversation step.