feat: complete phase 1 -- core framework with chat loop, agents, and React UI

Backend:
- FastAPI WebSocket /ws endpoint with streaming via LangGraph astream
- LangGraph Supervisor connecting 3 mock agents (order_lookup, order_actions, fallback)
- YAML Agent Registry with Pydantic validation and immutable configs
- PostgresSaver checkpoint persistence via langgraph-checkpoint-postgres
- Session TTL with 30-min sliding window and interrupt extension
- LLM provider abstraction (Anthropic/OpenAI/Google)
- Token usage + cost tracking callback handler
- Input validation: message size cap, thread_id format, content length
- Security: no hardcoded defaults, startup API key validation, no input reflection

Frontend:
- React 19 + TypeScript + Vite chat UI
- WebSocket hook with reconnect + exponential backoff
- Streaming token display with agent attribution
- Interrupt approval/reject UI for write operations
- Collapsible tool call viewer

Testing:
- 87 unit tests, 87% coverage (exceeds 80% requirement)
- Ruff lint + format clean

Infrastructure:
- Docker Compose (PostgreSQL 16 + backend)
- pyproject.toml with full dependency management

backend/app/agents/__init__.py

@@ -0,0 +1,30 @@
+"""Agent tools registry -- maps tool name strings to actual tool functions."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from langchain_core.tools import BaseTool
+
+from app.agents.fallback import fallback_respond
+from app.agents.order_actions import cancel_order
+from app.agents.order_lookup import get_order_status, get_tracking_info
+
+_TOOL_MAP: dict[str, BaseTool] = {
+    "get_order_status": get_order_status,
+    "get_tracking_info": get_tracking_info,
+    "cancel_order": cancel_order,
+    "fallback_respond": fallback_respond,
+}
+
+
+def get_tools_by_names(tool_names: list[str]) -> list[BaseTool]:
+    """Resolve tool name strings from YAML config to actual tool objects."""
+    tools = []
+    for name in tool_names:
+        if name not in _TOOL_MAP:
+            available = ", ".join(sorted(_TOOL_MAP.keys()))
+            raise ValueError(f"Unknown tool '{name}'. Available tools: {available}")
+        tools.append(_TOOL_MAP[name])
+    return tools