harness: fix double free of streamed chunk text (chunk payloads are borrowed per letta.zig contract)

onProgressEvent freed .chunk text that handleLine also frees via defer —
double free corrupted the heap mid-turn; DebugAllocator caught it and
the reply tail was silently lost (delivered messages missing their final
characters while the CLI transcript held the full text).

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta Code <noreply@letta.com>

src/matrix_harness.zig

@@ -360,7 +360,9 @@ fn onProgressEvent(ctx: ?*anyopaque, ev: letta.Event) void {
             p.ret(out);
         },
         .chunk => |c| {
-            defer p.alloc.free(c);
+            // NOTE: chunk/step payloads are BORROWED — handleLine frees them
+            // via defer (same contract as the overlay UI). Freeing here too
+            // is a double free (heap corruption, silently lost reply tails).
             _ = p.replyChunk(c, false);
         },
         .step => {}, // legacy pre-formatted lines: superseded by structured