feat: add context arguments to /compact command

[arsham]

Changes in this PR:

• Allow /compact slash command to accept optional arguments for summary focus
• Fix parentID bug where compaction didn't replace session properly
• Update compaction pipeline to pass context through all stages
• Update types and documentation

Possibly related to #2465

[rektide]

This feels a lot closer to Ampcode's /handoff, which I strongly enjoy. It takes what you say in /handoff and generates a new prompt for the handoff from that. It's very nice being able to guide the compaction to where you're going next!!
https://ampcode.com/news/handoff