feat: implement core/ and platform/ with full test coverage

- platform/interface.py: PlatformClient Protocol + Pydantic models (User,
  MessageResponse, UserSettings) — no explicit session management, Master
  handles container lifecycle
- platform/mock.py: MockPlatformClient with simulated latency, [MOCK]
  responses, is_new correctly True only on first creation
- core/protocol.py: unified dataclasses for all events and responses
  (IncomingMessage/Command/Callback, OutgoingMessage/UI/Notification,
  AuthFlow, ChatContext, SettingsAction, etc.)
- core/store.py: StateStore Protocol + InMemoryStore (tests) + SQLiteStore
  (prod) with JSON serialization
- core/chat.py: ChatManager — chat metadata (C1/C2/C3), not container
  lifecycle (that's the platform's job)
- core/auth.py: AuthManager — start_flow / confirm / is_authenticated
- core/settings.py: SettingsManager — get/apply with store cache
- core/handler.py: EventDispatcher — registry-based routing with keys
  (command name, action name, attachment type, "*" catch-all)
- core/handlers/: register_all() + start/new/message/callback/settings
  handlers; voice slot falls back to stub text until voice_handler added
- conftest.py: sys.path fix so local platform/ shadows stdlib platform
- docs/api-contract.md: rewritten for Lambda Lab 3.0 container model

46 tests passing, 0 warnings.

platform/interface.py

@@ -0,0 +1,59 @@
+# platform/interface.py
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Any, Protocol
+
+from pydantic import BaseModel
+
+
+class User(BaseModel):
+    user_id: str
+    external_id: str
+    platform: str
+    display_name: str | None = None
+    created_at: datetime
+    is_new: bool = False
+
+
+class MessageResponse(BaseModel):
+    message_id: str
+    response: str
+    tokens_used: int
+    finished: bool
+
+
+class UserSettings(BaseModel):
+    skills: dict[str, bool] = {}
+    connectors: dict[str, dict] = {}
+    soul: dict[str, str] = {}
+    safety: dict[str, bool] = {}
+    plan: dict[str, Any] = {}
+
+
+class PlatformError(Exception):
+    def __init__(self, message: str, code: str = "PLATFORM_ERROR"):
+        super().__init__(message)
+        self.code = code
+
+
+class PlatformClient(Protocol):
+    async def get_or_create_user(
+        self,
+        external_id: str,
+        platform: str,
+        display_name: str | None = None,
+    ) -> User: ...
+
+    # Master manages container lifecycle — bot only sends user_id + chat_id.
+    async def send_message(
+        self,
+        user_id: str,
+        chat_id: str,
+        text: str,
+        attachments: list | None = None,
+    ) -> MessageResponse: ...
+
+    async def get_settings(self, user_id: str) -> UserSettings: ...
+
+    async def update_settings(self, user_id: str, action: Any) -> None: ...