feat: add edge-tts as zero-cost voice output provider

- Add EdgeTTSAdapter to bin/deepdive_tts.py (provider key: "edge-tts")
  default voice: en-US-GuyNeural, no API key required
- Add EdgeTTS class to intelligence/deepdive/tts_engine.py
- Update HybridTTS to try edge-tts as fallback between piper and elevenlabs
- Add --voice-memo flag to bin/night_watch.py for spoken nightly reports
- Add edge-tts>=6.1.9 to requirements.txt
- Create docs/voice-output.md documenting all providers and fallback chain
- Add tests/test_edge_tts.py with 17 unit tests (all mocked, no network)

Fixes #1126

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/deepdive_tts.py

@@ -152,17 +152,55 @@ class OpenAITTSAdapter:
         return mp3_path
 
 
+class EdgeTTSAdapter:
+    """Zero-cost TTS using Microsoft Edge neural voices (no API key required).
+
+    Requires: pip install edge-tts>=6.1.9
+    Voices: https://learn.microsoft.com/en-us/azure/ai-services/speech-service/language-support
+    """
+
+    DEFAULT_VOICE = "en-US-GuyNeural"
+
+    def __init__(self, config: TTSConfig):
+        self.config = config
+        self.voice = config.voice_id or self.DEFAULT_VOICE
+
+    def synthesize(self, text: str, output_path: Path) -> Path:
+        try:
+            import edge_tts
+        except ImportError:
+            raise RuntimeError("edge-tts not installed. Run: pip install edge-tts")
+
+        import asyncio
+
+        mp3_path = output_path.with_suffix(".mp3")
+
+        async def _run():
+            communicate = edge_tts.Communicate(text, self.voice)
+            await communicate.save(str(mp3_path))
+
+        asyncio.run(_run())
+        return mp3_path
+
+
 ADAPTERS = {
     "piper": PiperAdapter,
     "elevenlabs": ElevenLabsAdapter,
     "openai": OpenAITTSAdapter,
+    "edge-tts": EdgeTTSAdapter,
 }
 
 
 def get_provider_config() -> TTSConfig:
     """Load TTS configuration from environment."""
     provider = os.environ.get("DEEPDIVE_TTS_PROVIDER", "openai")
-    voice = os.environ.get("DEEPDIVE_TTS_VOICE", "alloy" if provider == "openai" else "matthew")
+    if provider == "openai":
+        default_voice = "alloy"
+    elif provider == "edge-tts":
+        default_voice = EdgeTTSAdapter.DEFAULT_VOICE
+    else:
+        default_voice = "matthew"
+    voice = os.environ.get("DEEPDIVE_TTS_VOICE", default_voice)
     
     return TTSConfig(
         provider=provider,