feat: VoiceCoach v2 - OpenClaw gateway + Speaches STT/TTS

- pipeline.py: completely rewritten to use Speaches STT/TTS and OpenClaw LLM
- main.py: v2 with OPENCLAW_TOKEN support
- requirements.txt: slimmed (removed anthropic, faster-whisper direct deps)
- Dockerfile: slimmer build (no model pre-download)
- whisper_stt.py, piper_tts.py: removed (replaced by Speaches API)
- docker-compose.yml: v2 with speaches + openclaw services, piper-tts removed
- openclaw-agent/system_prompt.md: new directory with agent system prompt

pipecat-agent/piper_tts.py

@@ -1,34 +0,0 @@
-"""
-Piper TTS client for Pipecat pipeline.
-Calls the self-hosted Piper HTTP service.
-"""
-import io
-import logging
-import wave
-import httpx
-
-logger = logging.getLogger(__name__)
-
-
-class PiperTTSClient:
-    """HTTP client for the Piper TTS service."""
-
-    def __init__(self, base_url: str = "http://piper-tts:8003"):
-        self.base_url = base_url.rstrip("/")
-        self._client = httpx.AsyncClient(timeout=10.0)
-
-    async def synthesize(self, text: str) -> bytes:
-        """Convert text to WAV audio bytes via Piper service."""
-        try:
-            response = await self._client.post(
-                f"{self.base_url}/synthesize",
-                json={"text": text, "length_scale": 0.95, "sentence_silence": 0.15},
-            )
-            response.raise_for_status()
-            return response.content
-        except Exception as e:
-            logger.error(f"Piper TTS failed: {e}")
-            return b""
-
-    async def close(self):
-        await self._client.aclose()