fix(hub): fence close must reject info strings per CommonMark spec

The closing fence regex was not checking for an empty info string,
allowing e.g. ```python to incorrectly close an open fence. Also
adds missing test for tool_execution_update passthrough.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/hub/block-chunker.test.ts

@@ -223,6 +223,23 @@ describe("BlockChunker", () => {
       expect(chunk + remainder).toBe(text);
     });
 
+    it("does not treat fence with info string as closing fence", () => {
+      const chunker = new BlockChunker(cfg({ minChars: 10, maxChars: 500 }));
+      // The second ```python should NOT close the first fence (CommonMark: closing fence has no info string)
+      const text = "Before.\n\n```python\ncode line 1\n```python\ncode line 2\n```\n\nAfter text here.";
+      const result = chunker.tryChunk(text);
+      expect(result).not.toBeNull();
+      const chunk = result!.chunk;
+      // The split should not land between ```python and ```python (inside fence)
+      // It should either be before the fence or after the closing ```
+      const fenceOpens = (chunk.match(/```python/g) || []).length;
+      const fenceCloses = (chunk.match(/^```$/gm) || []).length;
+      if (fenceOpens > 0) {
+        // If chunk includes opening fences, it must include the real close
+        expect(fenceCloses).toBeGreaterThanOrEqual(1);
+      }
+    });
+
     it("handles multiple sequential code blocks", () => {
       const chunker = new BlockChunker(cfg({ minChars: 10, maxChars: 500 }));
       const text = "```js\nfoo()\n```\n\n```py\nbar()\n```\n\nEnd.";

src/hub/block-chunker.ts

@@ -51,8 +51,8 @@ function detectFenceAt(text: string, upTo: number): FenceInfo | null {
     if (openFence === null) {
       // Opening a new fence
       openFence = { marker, lang };
-    } else if (markerChar === openFence.marker[0] && marker.length >= openFence.marker.length) {
-      // Closing the current fence (same char, at least as many chars)
+    } else if (markerChar === openFence.marker[0] && marker.length >= openFence.marker.length && lang === "") {
+      // Closing the current fence (same char, at least as many chars, no info string per CommonMark)
       openFence = null;
     }
     // Otherwise: different char or shorter marker, not a close — ignore

src/hub/message-aggregator.test.ts

@@ -112,6 +112,14 @@ describe("MessageAggregator", () => {
       expect(onBlock).not.toHaveBeenCalled();
     });
 
+    it("passes through tool_execution_update immediately", () => {
+      const agg = new MessageAggregator(smallConfig(), onBlock, onPassthrough);
+      const event = { type: "tool_execution_update", toolCallId: "tool-1", content: "output" } as unknown as AgentEvent;
+      agg.handleEvent(event);
+      expect(onPassthrough).toHaveBeenCalledWith(event);
+      expect(onBlock).not.toHaveBeenCalled();
+    });
+
     it("passes through compaction_start immediately", () => {
       const agg = new MessageAggregator(smallConfig(), onBlock, onPassthrough);
       const event = makeCompactionStart();