docs: cover compaction notifyUser config and provider replay hooks

docs/gateway/configuration-reference.md

@@ -1064,6 +1064,7 @@ Periodic heartbeat runs.
         identifierInstructions: "Preserve deployment IDs, ticket IDs, and host:port pairs exactly.", // used when identifierPolicy=custom
         postCompactionSections: ["Session Startup", "Red Lines"], // [] disables reinjection
         model: "openrouter/anthropic/claude-sonnet-4-6", // optional compaction-only model override
+        notifyUser: true, // send a brief notice when compaction starts (default: false)
         memoryFlush: {
           enabled: true,
           softThresholdTokens: 6000,
@@ -1082,6 +1083,7 @@ Periodic heartbeat runs.
 - `identifierInstructions`: optional custom identifier-preservation text used when `identifierPolicy=custom`.
 - `postCompactionSections`: optional AGENTS.md H2/H3 section names to re-inject after compaction. Defaults to `["Session Startup", "Red Lines"]`; set `[]` to disable reinjection. When unset or explicitly set to that default pair, older `Every Session`/`Safety` headings are also accepted as a legacy fallback.
 - `model`: optional `provider/model-id` override for compaction summarization only. Use this when the main session should keep one model but compaction summaries should run on another; when unset, compaction uses the session's primary model.
+- `notifyUser`: when `true`, sends a brief notice to the user when compaction starts (for example, "Compacting context..."). Disabled by default to keep compaction silent.
 - `memoryFlush`: silent agentic turn before auto-compaction to store durable memories. Skipped when workspace is read-only.
 
 ### `agents.defaults.contextPruning`