fix(gateway): prevent duplicate messages on no-message-id platforms

Platforms that don't return a message_id after the first send (Signal,
GitHub webhooks) were causing GatewayStreamConsumer to re-enter the
"first send" path on every tool boundary, posting one platform message
per tool call (observed as 155 PR comments on a single response).

Fix: treat _message_id == "__no_edit__" as a sentinel meaning "platform
accepted the send but cannot be edited". When a tool boundary arrives
in that state, skip the message_id/accumulated/last_sent_text reset so
all continuation text is delivered once via _send_fallback_final rather
than re-posted per segment.

Also make prompt_toolkit imports in hermes_cli/commands.py optional so
gateway and test environments that lack the package can still import
resolve_command, gateway_help_lines, and COMMAND_REGISTRY.

tests/gateway/test_stream_consumer.py

@@ -437,6 +437,45 @@ class TestSegmentBreakOnToolBoundary:
         # Only one send call (the initial message)
         assert adapter.send.call_count == 1
 
+    @pytest.mark.asyncio
+    async def test_no_message_id_segment_breaks_do_not_resend(self):
+        """On a platform that never returns a message_id (e.g. webhook with
+        github_comment delivery), tool-call segment breaks must NOT trigger
+        a new adapter.send() per boundary.  The fix: _message_id == '__no_edit__'
+        suppresses the reset so all text accumulates and is sent once."""
+        adapter = MagicMock()
+        # No message_id on first send, then one more for the fallback final
+        adapter.send = AsyncMock(side_effect=[
+            SimpleNamespace(success=True, message_id=None),
+            SimpleNamespace(success=True, message_id=None),
+        ])
+        adapter.edit_message = AsyncMock(return_value=SimpleNamespace(success=True))
+        adapter.MAX_MESSAGE_LENGTH = 4096
+
+        config = StreamConsumerConfig(edit_interval=0.01, buffer_threshold=5)
+        consumer = GatewayStreamConsumer(adapter, "chat_123", config)
+
+        # Simulate: text → tool boundary → text → tool boundary → text (3 segments)
+        consumer.on_delta("Phase 1 text")
+        consumer.on_delta(None)   # tool call boundary
+        consumer.on_delta("Phase 2 text")
+        consumer.on_delta(None)   # another tool call boundary
+        consumer.on_delta("Phase 3 text")
+        consumer.finish()
+
+        await consumer.run()
+
+        # Before the fix this would post 3 comments (one per segment).
+        # After the fix: only the initial partial + one fallback-final continuation.
+        assert adapter.send.call_count == 2, (
+            f"Expected 2 sends (initial + fallback), got {adapter.send.call_count}"
+        )
+        assert consumer.already_sent
+        # The continuation must contain the text from segments 2 and 3
+        final_text = adapter.send.call_args_list[1][1]["content"]
+        assert "Phase 2" in final_text
+        assert "Phase 3" in final_text
+
     @pytest.mark.asyncio
     async def test_fallback_final_splits_long_continuation_without_dropping_text(self):
         """Long continuation tails should be chunked when fallback final-send runs."""