Summary

Extends the experimental.session.compacting hook to support full prompt replacement via output.prompt.

This was explicitly called out by @rekram1-node in #5698:

I imagine there are people who'd wanna override the instructions instead of just append and this hook doesn't support that but we prolly wouldn't want a brand new one

Changes

Hook Behavior

• output.context - appends to default prompt (unchanged)
• output.prompt - replaces entire prompt (context ignored when set)

Example

"experimental.session.compacting": async (input, output) => {
  output.prompt = `
You are generating a continuation prompt for a multi-agent swarm session.
Summarize the current task, file reservations, and next steps.
`
}