docs(plugin): list compaction hook result fields - #57276

Problem

The plugin docs said a compaction hook's result "takes the same fields as a completed compaction message". That isn't true: it takes summary, metadata, native and tokens, and not reason, model or cost. They also didn't mention the two keys v2.1 gives meaning to: metadata.keepID and native.messages.

Change

The example now lists the optional result fields in comments, the way the other hook examples on that page keep internals out of the prose:

event.result = {
  summary: await summarize(event.messages),
  // Optional:
  // metadata: { keepID } - first message to keep verbatim after the summary; defaults to OpenCode's choice
  // native: { messages } - a provider replacement window, replayed instead of `summary` on the same model
  // tokens - usage of your summary request, included in session totals
}

Out of scope

packages/web/.../plugins.mdx (19 locales) lists session.compacted. Those pages document V1 plugins, where that event exists, so they stay as they are.

Verification

• bunx prettier --check on the page: clean.
• Checked each comment against packages/core/src/session/compaction.ts:
  • { keepID: split.keepID, ...result.metadata } lets the hook's value win;
  • fromHook records tokens through spend;
  • replay requires the compaction's provider ID and model ID to match.