Add optional user message to /compact, alter prompt

What feature would you like to see?

Sometimes it is preferable to provide direction on which parts of the interaction to prioritize while summarizing. A small optional user message guiding the summarization would be very helpful.

Additionally, I have noticed the agent sometimes refuses to provide a summary for shorter interactions. Framing the prompts in compact/prompt.md and compact/history_bridge.md with 'yourself'/'you' instead of 'next agent'/'another agent' works better, also seems like a more natural approach.

Additional information

_No response_