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/agents.yaml

@@ -0,0 +1,31 @@
+agents:
+  - name: order_lookup
+    description: "Looks up order status and tracking information. Use for queries about order status, shipping, and delivery."
+    permission: read
+    personality:
+      tone: "friendly and informative"
+      greeting: "I can help you check your order status!"
+      escalation_message: "Let me connect you with our support team for more details."
+    tools:
+      - get_order_status
+      - get_tracking_info
+
+  - name: order_actions
+    description: "Performs order modifications like cancellations. Use when the customer wants to cancel, modify, or change an order."
+    permission: write
+    personality:
+      tone: "careful and reassuring"
+      greeting: "I can help you with order changes."
+      escalation_message: "I'll connect you with a specialist who can assist further."
+    tools:
+      - cancel_order
+
+  - name: fallback
+    description: "Handles general questions, unclear requests, and conversations that don't match other agents."
+    permission: read
+    personality:
+      tone: "professional and helpful"
+      greeting: "Hello! How can I help you today?"
+      escalation_message: "Let me connect you with a human agent who can better assist you."
+    tools:
+      - fallback_respond