feat: scaffold watch-screen AI pipeline (mock-runnable skeleton)

Modular async pipeline: FrameSource->Vision->context and STT/text->Brain->TTS.
All stages are Protocols; mock backends run end-to-end with no deps/keys.
Real backends included: mss screen capture, Claude vision+brain (guarded imports).

wsai/factory.py

@@ -0,0 +1,70 @@
+"""Build a Pipeline from Settings. This is the single place that knows which
+concrete class each config name maps to, so adding a backend = one line here."""
+
+from __future__ import annotations
+
+from .config import Settings
+from .pipeline import Pipeline
+
+
+def build(settings: Settings) -> Pipeline:
+    return Pipeline(
+        source=_source(settings),
+        vision=_vision(settings),
+        brain=_brain(settings),
+        stt=_stt(settings),
+        tts=_tts(settings),
+        text_channel=_text(settings),
+    )
+
+
+def _source(s: Settings):
+    if s.source == "mss":
+        from .backends.capture_mss import MSSFrameSource
+
+        return MSSFrameSource(interval=s.capture_interval)
+    from .backends.mock import MockFrameSource
+
+    return MockFrameSource(interval=s.capture_interval)
+
+
+def _vision(s: Settings):
+    if s.vision == "claude":
+        from .backends.claude import ClaudeVision
+
+        return ClaudeVision(model=s.anthropic_model)
+    from .backends.mock import MockVision
+
+    return MockVision()
+
+
+def _brain(s: Settings):
+    if s.brain == "claude":
+        from .backends.claude import ClaudeBrain
+
+        return ClaudeBrain(model=s.anthropic_model)
+    from .backends.mock import MockBrain
+
+    return MockBrain()
+
+
+def _stt(s: Settings):
+    if s.stt in (None, "none"):
+        return None
+    from .backends.mock import MockSTT
+
+    return MockSTT()
+
+
+def _tts(s: Settings):
+    if s.tts in (None, "none"):
+        return None
+    from .backends.mock import MockTTS
+
+    return MockTTS()
+
+
+def _text(s: Settings):
+    if s.text in (None, "none"):
+        return None
+    raise NotImplementedError("discord text channel backend not implemented yet")