fix(session): prevent silent overflow on parent thread forks (#26912)

Lands #26912 from @markshields-tl with configurable session.parentForkMaxTokens and docs/tests/changelog updates.

Co-authored-by: Mark Shields <239231357+markshields-tl@users.noreply.github.com>

src/auto-reply/reply/agent-runner-execution.ts

@@ -572,6 +572,22 @@ export async function runAgentTurnWithFallback(params: {
     }
   }
 
+  // If the run completed but with an embedded context overflow error that
+  // wasn't recovered from (e.g. compaction reset already attempted), surface
+  // the error to the user instead of silently returning an empty response.
+  // See #26905: Slack DM sessions silently swallowed messages when context
+  // overflow errors were returned as embedded error payloads.
+  const finalEmbeddedError = runResult?.meta?.error;
+  const hasPayloadText = runResult?.payloads?.some((p) => p.text?.trim());
+  if (finalEmbeddedError && isContextOverflowError(finalEmbeddedError.message) && !hasPayloadText) {
+    return {
+      kind: "final",
+      payload: {
+        text: "⚠️ Context overflow — this conversation is too large for the model. Use /new to start a fresh session.",
+      },
+    };
+  }
+
   return {
     kind: "success",
     runId,