feat: send compaction start and completion notices (#67830)

Merged via squash.

Prepared head SHA: abedf6cf11
Co-authored-by: feniix <91633+feniix@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

docs/.generated/config-baseline.sha256

@@ -1,4 +1,4 @@
-747a24d0acf12f95ec75feabb47dad8f03ff0e3a7173b4d277c648f75d956ce5  config-baseline.json
-cbb9a6ee1cb69068d5eb63f00f95512ba19778415ea5b2eabe056aaea38978b5  config-baseline.core.json
+ab40431597e9f7c09a9f010f267bab250c7f9c570c4a100776de98869f589a92  config-baseline.json
+04a82c2208bf69e0a195e7712e3a518a8255c1bb002c31f712cb95003325635b  config-baseline.core.json
 e239cc20f20f8d0172812bc0ad3ee6df52da88e2e2702e3d03a47e01561132ae  config-baseline.channel.json
 b695cb31b4c0cf1d31f842f2892e99cc3ff8d84263ae72b72977cae844b81d6e  config-baseline.plugin.json

docs/concepts/compaction.md

@@ -132,10 +132,10 @@ capable model for better summaries:
 }
 ```
 
-## Compaction start notice
+## Compaction notices
 
-By default, compaction runs silently. To show a brief notice when compaction
-starts, enable `notifyUser`:
+By default, compaction runs silently. To show brief notices when compaction
+starts and when it completes, enable `notifyUser`:
 
 ```json5
 {
@@ -149,8 +149,8 @@ starts, enable `notifyUser`:
 }
 ```
 
-When enabled, the user sees a short message (for example, "Compacting
-context...") at the start of each compaction run.
+When enabled, the user sees short status messages around each compaction run
+(for example, "Compacting context..." and "Compaction complete").
 
 ## Compaction vs pruning
 

docs/gateway/configuration-reference.md

@@ -1392,7 +1392,7 @@ 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)
+        notifyUser: true, // send brief notices when compaction starts and completes (default: false)
         memoryFlush: {
           enabled: true,
           softThresholdTokens: 6000,
@@ -1412,7 +1412,7 @@ 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.
+- `notifyUser`: when `true`, sends brief notices to the user when compaction starts and when it completes (for example, "Compacting context..." and "Compaction complete"). 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`