feat: ship comfyui_music as a custom-workflow-only ACE-Step tool

Resolves the "music generation" open question from the adapter plan.
Unlike comfyui_image/comfyui_video there is no bundled workflow: ACE-Step's
ComfyUI node interface isn't standardized across custom node packs
(AceStepModelLoader vs native TextEncodeAceStepAudio, etc.), so instead of
picking one pack and breaking for everyone else, comfyui_music always
requires a caller-supplied workflow_json/workflow_path + output_node --
the same override contract image/video offer as an alternative, just
mandatory here. prompt is provenance-only, never injected into the graph.

Routed through the existing registry.get_by_capability("music_generation")
path alongside suno_music/music_gen -- no dedicated selector needed.
ComfyUIClient.generate() now also reads the "audio" output key (what
ComfyUI's native SaveAudio node writes), and gets timeout/resume/websocket-
wait/multi-server support for free via the shared client. Duration is a
best-effort ffprobe probe of the downloaded file since a custom workflow
gives no other way to know it ahead of time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/_comfyui/client.py

@@ -404,8 +404,13 @@ class ComfyUIClient:
         outputs = entry.get("outputs", {})
         node_output = outputs.get(output_node, {})
 
-        # ComfyUI stores images and videos under the "images" key
-        items = node_output.get("images", []) or node_output.get("gifs", [])
+        # ComfyUI stores images/video frames under "images", legacy GIFs
+        # under "gifs", and the native SaveAudio node's output under "audio".
+        items = (
+            node_output.get("images", [])
+            or node_output.get("gifs", [])
+            or node_output.get("audio", [])
+        )
         if not items:
             raise ComfyUIError(
                 f"No output artifacts on node {output_node}. "

tools/_comfyui/metadata.py

@@ -24,6 +24,7 @@ COMFYUI_SETUP_OFFER: dict[str, Any] = {
     "per_capability_env_var_overrides": {
         "comfyui_image": "COMFYUI_IMAGE_SERVER_URL",
         "comfyui_video": "COMFYUI_VIDEO_SERVER_URL",
+        "comfyui_music": "COMFYUI_MUSIC_SERVER_URL",
     },
 }