fix(heartbeat): prune transcript for HEARTBEAT_OK turns

When a heartbeat run results in HEARTBEAT_OK (or empty/duplicate), the user+assistant
turns are now pruned from the session transcript. This prevents context window
pollution from zero-information exchanges.

Implementation:
- captureTranscriptState(): records transcript file path and size before heartbeat
- pruneHeartbeatTranscript(): truncates file back to pre-heartbeat size
- Called in ok-empty, ok-token, and duplicate cases (same places as restoreHeartbeatUpdatedAt)

This extends the existing pattern where delivery is suppressed and updatedAt is restored
for HEARTBEAT_OK responses - now the transcript is also cleaned up.

Fixes #17804

src/infra/heartbeat-runner.ts

@@ -31,6 +31,7 @@ import {
   loadSessionStore,
   resolveAgentIdFromSessionKey,
   resolveAgentMainSessionKey,
+  resolveSessionFilePath,
   resolveStorePath,
   saveSessionStore,
   updateSessionStore,
@@ -351,6 +352,58 @@ async function restoreHeartbeatUpdatedAt(params: {
   });
 }
 
+/**
+ * Prune heartbeat transcript entries by truncating the file back to a previous size.
+ * This removes the user+assistant turns that were written during a HEARTBEAT_OK run,
+ * preventing context pollution from zero-information exchanges.
+ */
+async function pruneHeartbeatTranscript(params: {
+  transcriptPath?: string;
+  preHeartbeatSize?: number;
+}) {
+  const { transcriptPath, preHeartbeatSize } = params;
+  if (!transcriptPath || typeof preHeartbeatSize !== "number" || preHeartbeatSize < 0) {
+    return;
+  }
+  try {
+    const stat = await fs.stat(transcriptPath);
+    // Only truncate if the file has grown during the heartbeat run
+    if (stat.size > preHeartbeatSize) {
+      await fs.truncate(transcriptPath, preHeartbeatSize);
+    }
+  } catch {
+    // File may not exist or may have been removed - ignore errors
+  }
+}
+
+/**
+ * Get the transcript file path and its current size before a heartbeat run.
+ * Returns undefined values if the session or transcript doesn't exist yet.
+ */
+async function captureTranscriptState(params: {
+  storePath: string;
+  sessionKey: string;
+  agentId?: string;
+}): Promise<{ transcriptPath?: string; preHeartbeatSize?: number }> {
+  const { storePath, sessionKey, agentId } = params;
+  try {
+    const store = loadSessionStore(storePath);
+    const entry = store[sessionKey];
+    if (!entry?.sessionId) {
+      return {};
+    }
+    const transcriptPath = resolveSessionFilePath(entry.sessionId, entry, {
+      agentId,
+      sessionsDir: path.dirname(storePath),
+    });
+    const stat = await fs.stat(transcriptPath);
+    return { transcriptPath, preHeartbeatSize: stat.size };
+  } catch {
+    // Session or transcript doesn't exist yet - nothing to prune
+    return {};
+  }
+}
+
 function normalizeHeartbeatReply(
   payload: ReplyPayload,
   responsePrefix: string | undefined,
@@ -546,6 +599,13 @@ export async function runHeartbeatOnce(opts: {
   };
 
   try {
+    // Capture transcript state before the heartbeat run so we can prune if HEARTBEAT_OK
+    const transcriptState = await captureTranscriptState({
+      storePath,
+      sessionKey,
+      agentId,
+    });
+
     const heartbeatModelOverride = heartbeat?.model?.trim() || undefined;
     const suppressToolErrorWarnings = heartbeat?.suppressToolErrorWarnings === true;
     const replyOpts = heartbeatModelOverride
@@ -567,6 +627,8 @@ export async function runHeartbeatOnce(opts: {
         sessionKey,
         updatedAt: previousUpdatedAt,
       });
+      // Prune the transcript to remove HEARTBEAT_OK turns
+      await pruneHeartbeatTranscript(transcriptState);
       const okSent = await maybeSendHeartbeatOk();
       emitHeartbeatEvent({
         status: "ok-empty",
@@ -601,6 +663,8 @@ export async function runHeartbeatOnce(opts: {
         sessionKey,
         updatedAt: previousUpdatedAt,
       });
+      // Prune the transcript to remove HEARTBEAT_OK turns
+      await pruneHeartbeatTranscript(transcriptState);
       const okSent = await maybeSendHeartbeatOk();
       emitHeartbeatEvent({
         status: "ok-token",
@@ -637,6 +701,8 @@ export async function runHeartbeatOnce(opts: {
         sessionKey,
         updatedAt: previousUpdatedAt,
       });
+      // Prune the transcript to remove duplicate heartbeat turns
+      await pruneHeartbeatTranscript(transcriptState);
       emitHeartbeatEvent({
         status: "skipped",
         reason: "duplicate",