feat(plugin): allow compaction hook to replace prompt entirely

[joelhooks]

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

File	Change
packages/plugin/src/index.ts	Add optional prompt?: string to hook output type
packages/opencode/src/session/compaction.ts	Use output.prompt if set, otherwise default + context
packages/web/src/content/docs/plugins.mdx	Document prompt replacement usage

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.
`
}

[joelhooks]

@rekram1-node that was a good point about wanting to replace the prompt, I actually do 😅

this makes it a lot more robust.

[rekram1-node]

/review

[github-actions]

lgtm

[joelhooks]

@rekram1-node need anything from me here? this one is a huge unlock for me and a better implementation than the first pass 🙏