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>

intelligence/deepdive/tts_engine.py

@@ -157,14 +157,45 @@ class ElevenLabsTTS:
         return output_path
 
 
+class EdgeTTS:
+    """Zero-cost TTS using Microsoft Edge neural voices (no API key required).
+
+    Requires: pip install edge-tts>=6.1.9
+    """
+
+    DEFAULT_VOICE = "en-US-GuyNeural"
+
+    def __init__(self, voice: str = None):
+        self.voice = voice or self.DEFAULT_VOICE
+
+    def synthesize(self, text: str, output_path: str) -> str:
+        """Convert text to MP3 via Edge TTS."""
+        try:
+            import edge_tts
+        except ImportError:
+            raise RuntimeError("edge-tts not installed. Run: pip install edge-tts")
+
+        import asyncio
+        from pathlib import Path
+
+        mp3_path = str(Path(output_path).with_suffix(".mp3"))
+
+        async def _run():
+            communicate = edge_tts.Communicate(text, self.voice)
+            await communicate.save(mp3_path)
+
+        asyncio.run(_run())
+        return mp3_path
+
+
 class HybridTTS:
     """TTS with sovereign primary, cloud fallback."""
-    
+
     def __init__(self, prefer_cloud: bool = False):
         self.primary = None
         self.fallback = None
         self.prefer_cloud = prefer_cloud
-        
+
         # Try preferred engine
         if prefer_cloud:
             self._init_elevenlabs()
@@ -172,21 +203,29 @@ class HybridTTS:
                 self._init_piper()
         else:
             self._init_piper()
+            if not self.primary:
+                self._init_edge_tts()
             if not self.primary:
                 self._init_elevenlabs()
-    
+
     def _init_piper(self):
         try:
             self.primary = PiperTTS()
         except Exception as e:
             print(f"Piper init failed: {e}")
-    
+
+    def _init_edge_tts(self):
+        try:
+            self.primary = EdgeTTS()
+        except Exception as e:
+            print(f"EdgeTTS init failed: {e}")
+
     def _init_elevenlabs(self):
         try:
             self.primary = ElevenLabsTTS()
         except Exception as e:
             print(f"ElevenLabs init failed: {e}")
-    
+
     def synthesize(self, text: str, output_path: str) -> str:
         """Synthesize with fallback."""
         if self.primary:
@@ -194,7 +233,7 @@ class HybridTTS:
                 return self.primary.synthesize(text, output_path)
             except Exception as e:
                 print(f"Primary failed: {e}")
-        
+
         raise RuntimeError("No TTS engine available")