feat(voice): run pipeline eyes-free (STT -> Brain -> TTS), defer screen share

Make source/vision optional so the conversation loop runs with no screen
capture. Add Settings.voice() preset and `python -m wsai --voice` demo.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

wsai/pipeline.py

@@ -26,15 +26,16 @@ class Pipeline:
     * perception: FrameSource -> VisionBackend -> SharedScreenContext
     * conversation: (SpeechToText | TextChannel) -> Brain -> (TextToSpeech | TextChannel)
 
-    Either input/output half can be None, so you can run text-only, voice-only,
-    or a headless "just watch" configuration.
+    Any half can be omitted. With no source/vision it runs eyes-free as a pure
+    voice loop (STT -> Brain -> TTS); with no stt/tts it runs text-only; with no
+    conversation it is a headless "just watch" configuration.
     """
 
     def __init__(
         self,
         *,
-        source: FrameSource,
-        vision: VisionBackend,
+        source: FrameSource | None = None,
+        vision: VisionBackend | None = None,
         brain: Brain,
         stt: SpeechToText | None = None,
         tts: TextToSpeech | None = None,
@@ -53,6 +54,8 @@ class Pipeline:
 
     # -- perception -------------------------------------------------------- #
     async def _perceive(self) -> None:
+        if self.source is None or self.vision is None:
+            return  # eyes-free (voice-only) configuration
         async for frame in self.source.frames():
             try:
                 obs = await self.vision.describe(frame)