[FEATURE]: ACP: expose compaction summary marker so chat clients can suppress compaction dumps

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Describe the enhancement

Chat clients connected via the ACP protocol (e.g. opencode-chat-bridge, which forwards ACP updates to Matrix/Telegram) cannot distinguish an auto-compaction summary from a normal assistant reply.

OpenCode already marks the summary internally:

• the summary assistant message is created with summary: true and mode: "compaction"
• the auto-continue follow-up text part has metadata: { compaction_continue: true }
• a session.compacted event is published when compaction completes

But packages/opencode/src/acp/event.ts only handles permission.asked, message.part.updated, message.part.delta and forwards plain agent_message_chunk updates with no message-level info. The session.compacted event is never forwarded to ACP clients. As a result, chat users see an unexplained wall of text when the session auto-compacts.

Suggested fix: include message info (summary, mode) in the agent_message_chunk sessionUpdate payload (and in session/load replay), and/or forward the session.compacted event as a dedicated ACP notification.