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/backends/mock.py

@@ -0,0 +1,99 @@
+"""Mock backends. These let the full pipeline run with no GPU, no mic, no API
+key — so the skeleton is verifiable and gives every real backend a reference
+implementation to match.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import itertools
+import time
+from typing import AsyncIterator
+
+from ..interfaces import (
+    Frame,
+    Reply,
+    ScreenObservation,
+    Utterance,
+)
+
+
+class MockFrameSource:
+    """Emits tiny synthetic frames on a fixed interval."""
+
+    def __init__(self, interval: float = 1.0, limit: int | None = None) -> None:
+        self.interval = interval
+        self.limit = limit
+
+    async def frames(self) -> AsyncIterator[Frame]:
+        for i in itertools.count():
+            if self.limit is not None and i >= self.limit:
+                return
+            yield Frame(
+                data=b"\x89PNG\r\n\x1a\n",  # PNG magic; enough for a stub
+                width=1280,
+                height=720,
+                ts=time.monotonic(),
+                mime="image/png",
+            )
+            await asyncio.sleep(self.interval)
+
+    async def aclose(self) -> None:  # nothing to release
+        return
+
+
+class MockVision:
+    """Pretends to read the screen. Cycles through a few canned scenes."""
+
+    SCENES = [
+        "VS Code is open with a Python file; a traceback is visible in the terminal.",
+        "A browser shows a GitHub pull request diff.",
+        "A game is running; the player is in a menu screen.",
+    ]
+
+    def __init__(self) -> None:
+        self._i = 0
+
+    async def describe(self, frame: Frame, hint: str | None = None) -> ScreenObservation:
+        scene = self.SCENES[self._i % len(self.SCENES)]
+        self._i += 1
+        return ScreenObservation(text=scene, ts=frame.ts)
+
+
+class MockSTT:
+    """Feeds a scripted set of user utterances, then goes quiet."""
+
+    def __init__(self, script: list[str] | None = None, interval: float = 2.0) -> None:
+        self.script = script or [
+            "지금 화면에 뭐 보여?",
+            "저 에러 왜 나는 거야?",
+            "고마워",
+        ]
+        self.interval = interval
+
+    async def utterances(self) -> AsyncIterator[Utterance]:
+        for line in self.script:
+            await asyncio.sleep(self.interval)
+            yield Utterance(text=line, ts=time.monotonic(), source="voice")
+
+    async def aclose(self) -> None:
+        return
+
+
+class MockTTS:
+    """'Speaks' by printing. Real TTS swaps in here."""
+
+    async def speak(self, reply: Reply) -> None:
+        print(f"[TTS] {reply.text}")
+
+
+class MockBrain:
+    """Echo-style brain that references the current screen, so you can see the
+    screen context actually reaching the conversation loop."""
+
+    async def respond(self, user_text, screen, history) -> Reply:
+        seen = screen.text if screen else "아직 화면을 못 읽었어요"
+        return Reply(
+            text=f'(화면: "{seen}") 라고 봤어요. 말씀하신 "{user_text}"에 대해 답하자면… [mock]',
+            ts=time.monotonic(),
+        )