fix(box): always advertise outbox path in exec guidance

Outbound attachment collection (pipeline wrapper) runs on every turn
regardless of inbound files, but the agent was only told the per-query
outbox path inside the inbound-attachment note in LocalAgentRunner. So on
pure-generation turns (e.g. "generate a QR code"/chart/mermaid where the
user sent no file), the agent never learned the outbox path or the
query_id, wrote the generated file nowhere deliverable, and it was
silently dropped.

Move the outbox instruction into BoxService.get_system_guidance(query_id),
which is injected as a system message on every turn the exec tool is
available. The inbound note keeps its own (now redundant but harmless)
outbox line. Add unit tests asserting the outbox path is present with a
query_id and absent without one.

tests/unit_tests/box/test_box_service.py

@@ -546,6 +546,41 @@ async def test_box_service_rejects_host_mount_outside_allowed_roots(tmp_path):
         )
 
 
+class TestGetSystemGuidance:
+    """``get_system_guidance`` must ALWAYS advertise the per-query outbox path
+    when given a ``query_id`` — even with no inbound attachment — so files the
+    agent generates (QR codes, charts, rendered docs) are actually delivered.
+
+    The wrapper collects the outbox on every turn regardless of inbound files;
+    before this, the agent was only told the outbox path inside the
+    inbound-attachment note, so pure-generation turns produced files that were
+    silently dropped.
+    """
+
+    def _service(self, logger=None):
+        logger = logger or Mock()
+        runtime = BoxRuntime(logger=logger, backends=[FakeBackend(logger)], session_ttl_sec=300)
+        return BoxService(make_app(logger), client=_InProcessBoxRuntimeClient(logger, runtime))
+
+    def test_guidance_includes_outbox_when_query_id_given(self):
+        service = self._service()
+        guidance = service.get_system_guidance(42)
+        assert f'{service.OUTBOX_MOUNT_DIR}/42' in guidance
+        assert 'delivered to the user automatically' in guidance
+
+    def test_guidance_omits_outbox_without_query_id(self):
+        service = self._service()
+        guidance = service.get_system_guidance()
+        assert service.OUTBOX_MOUNT_DIR not in guidance
+        # core exec guidance is still present
+        assert 'exec tool' in guidance
+
+    def test_guidance_outbox_independent_of_inbound_attachments(self):
+        # A bare query_id (the pure-generation case) still gets the outbox note.
+        service = self._service()
+        assert f'{service.OUTBOX_MOUNT_DIR}/0' in service.get_system_guidance(0)
+
+
 @pytest.mark.asyncio
 async def test_box_runtime_rejects_host_mount_conflict_in_same_session(tmp_path):
     logger = Mock()