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/factory.py

@@ -19,6 +19,8 @@ def build(settings: Settings) -> Pipeline:
 
 
 def _source(s: Settings):
+    if s.source in (None, "none"):
+        return None
     if s.source == "mss":
         from .backends.capture_mss import MSSFrameSource
 
@@ -29,6 +31,8 @@ def _source(s: Settings):
 
 
 def _vision(s: Settings):
+    if s.vision in (None, "none"):
+        return None
     if s.vision == "claude":
         from .backends.claude import ClaudeVision