Implement Python email agent with multi-model AI support

Adds a complete email-based registration agent for Spielgruppe Pumuckl
based on the OpenSpec define-project-scope specifications.

Architecture
- Channel-agnostic EmailAgent core — no email-specific code in business logic
- Pluggable AI provider layer: Anthropic (Claude) and OpenAI (GPT) supported
  via a shared LLMProvider interface; switch with AI_PROVIDER env var
- IMAP polling for inbound emails with thread-tracking via email headers
  (Message-ID / In-Reply-To / References)
- SMTP for outbound replies and admin notifications
- File-based JSON storage for conversation state and completed registrations
- Admin-editable knowledge-base loaded from markdown files at startup

Key files
  src/config.py                  — env-var configuration
  src/providers/base.py          — abstract LLMProvider
  src/providers/anthropic_provider.py — Claude backend
  src/providers/openai_provider.py    — OpenAI backend
  src/agent/core.py              — EmailAgent orchestrator
  src/agent/prompts.py           — system prompt builder (KB + registration state)
  src/models/registration.py     — RegistrationData matching the JSON schema
  src/models/conversation.py     — ConversationState persisted per thread
  src/channels/email_channel.py  — IMAP/SMTP I/O + quoted-text stripping
  src/storage/json_store.py      — conversation & registration persistence
  src/notifications/notifier.py  — admin notification routing by playgroup type
  main.py                        — polling entry point
  requirements.txt               — anthropic, openai, python-dotenv, jsonschema
  .env.example                   — configuration template

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

src/models/conversation.py

@@ -0,0 +1,76 @@
+"""Conversation state model — persisted per email thread."""
+
+from dataclasses import dataclass, field
+from datetime import datetime, timezone
+from typing import Optional
+
+from .registration import RegistrationData
+
+
+def _now() -> str:
+    return datetime.now(timezone.utc).isoformat()
+
+
+@dataclass
+class ChatMessage:
+    role: str      # "user" or "assistant"
+    content: str
+    timestamp: str = field(default_factory=_now)
+
+
+@dataclass
+class ConversationState:
+    conversation_id: str
+    language: str = "de"                           # "de" or "en"
+    flow_step: str = "greeting"                    # current step in registration flow
+    registration: RegistrationData = field(default_factory=RegistrationData)
+    messages: list = field(default_factory=list)   # list[ChatMessage]
+    parent_email: str = ""
+    parent_name: Optional[str] = None
+    created_at: str = field(default_factory=_now)
+    updated_at: str = field(default_factory=_now)
+    last_activity: str = field(default_factory=_now)
+    completed: bool = False
+    reminder_count: int = 0
+
+    def to_dict(self) -> dict:
+        return {
+            "conversation_id": self.conversation_id,
+            "language": self.language,
+            "flow_step": self.flow_step,
+            "registration": self.registration.to_dict(),
+            "messages": [
+                {"role": m.role, "content": m.content, "timestamp": m.timestamp}
+                for m in self.messages
+            ],
+            "parent_email": self.parent_email,
+            "parent_name": self.parent_name,
+            "created_at": self.created_at,
+            "updated_at": self.updated_at,
+            "last_activity": self.last_activity,
+            "completed": self.completed,
+            "reminder_count": self.reminder_count,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "ConversationState":
+        state = cls(conversation_id=data["conversation_id"])
+        state.language = data.get("language", "de")
+        state.flow_step = data.get("flow_step", "greeting")
+        state.registration = RegistrationData.from_dict(data.get("registration", {}))
+        state.messages = [
+            ChatMessage(
+                role=m["role"],
+                content=m["content"],
+                timestamp=m.get("timestamp", ""),
+            )
+            for m in data.get("messages", [])
+        ]
+        state.parent_email = data.get("parent_email", "")
+        state.parent_name = data.get("parent_name")
+        state.created_at = data.get("created_at", "")
+        state.updated_at = data.get("updated_at", "")
+        state.last_activity = data.get("last_activity", "")
+        state.completed = data.get("completed", False)
+        state.reminder_count = data.get("reminder_count", 0)
+        return state