Initial project structure

Scaffold all modules, route stubs, data models, and config.
No logic implemented yet — all core methods raise NotImplementedError.
Establishes the full directory layout matching the architecture in CLAUDE.md.

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

fellowship/core/turn_engine.py

@@ -0,0 +1,48 @@
+"""
+Turn engine — constructs a bot's prompt and executes its LLM call.
+Handles rectification slot reservation and filling.
+"""
+
+import logging
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from fellowship.core.session import Session, BotConfig, Message
+
+logger = logging.getLogger(__name__)
+
+
+class TurnEngine:
+    def __init__(self, session: "Session") -> None:
+        self.session = session
+
+    async def execute_turn(self, bot: "BotConfig") -> "Message":
+        """
+        Full turn pipeline for one bot:
+          1. Reserve a rectification slot in history.
+          2. Assemble the bot's prompt (global system + bot system + context).
+          3. Call the LLM.
+          4. Fill the reserved slot with the response.
+          5. Return the completed Message.
+        """
+        raise NotImplementedError
+
+    def _reserve_slot(self, bot: "BotConfig") -> int:
+        """
+        Append a placeholder Message (content=None) to history and return its index.
+        This is the rectification slot — talker messages arriving during generation
+        are inserted after this index.
+        """
+        raise NotImplementedError
+
+    def _fill_slot(self, index: int, content: str, tokens: int) -> None:
+        """Fill a previously reserved slot with the completed bot response."""
+        raise NotImplementedError
+
+    def _build_prompt(self, bot: "BotConfig") -> list[dict]:
+        """
+        Assemble the messages list for the LLM call:
+          - System message: global_system_prompt + bot system_prompt
+          - User/assistant messages from context (messages only, no foreign system prompts)
+        """
+        raise NotImplementedError