feat(returnPrompt)+fix(task): add custom return instructions to the task tool - fix agent conservation in main session on result

[spoons-and-mirrors]

the returnPrompt is like a continuation prompt - the subtask can tell the parent session what to do next with its results and keep the agent loop going. it's a more automated command

It makes commands more powerful and augments it with a 2 step workflow capability with a single subtask command which uses the main session agent as a judge of the subtask or w/e you want to make it with the return instructions

This PR also fixes the subtask result message to use the main session agent instead of the child session agent.