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/tests/conftest.py

@@ -0,0 +1,69 @@
+"""Shared test fixtures and marker registration."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+import yaml
+
+from app.config import Settings
+from app.registry import AgentRegistry
+from app.session_manager import SessionManager
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+@pytest.fixture
+def test_settings() -> Settings:
+    return Settings(
+        database_url="postgresql://test:test@localhost:5432/test_db",
+        llm_provider="anthropic",
+        llm_model="claude-sonnet-4-6",
+        anthropic_api_key="test-key",
+    )
+
+
+@pytest.fixture
+def sample_yaml_path(tmp_path: Path) -> Path:
+    data = {
+        "agents": [
+            {
+                "name": "test_reader",
+                "description": "A test read agent",
+                "permission": "read",
+                "tools": ["get_order_status"],
+            },
+            {
+                "name": "test_writer",
+                "description": "A test write agent",
+                "permission": "write",
+                "personality": {
+                    "tone": "formal",
+                    "greeting": "Greetings.",
+                    "escalation_message": "Escalating now.",
+                },
+                "tools": ["cancel_order"],
+            },
+            {
+                "name": "test_fallback",
+                "description": "A fallback agent",
+                "permission": "read",
+                "tools": ["fallback_respond"],
+            },
+        ]
+    }
+    path = tmp_path / "test_agents.yaml"
+    path.write_text(yaml.dump(data), encoding="utf-8")
+    return path
+
+
+@pytest.fixture
+def sample_registry(sample_yaml_path: Path) -> AgentRegistry:
+    return AgentRegistry.load(sample_yaml_path)
+
+
+@pytest.fixture
+def session_manager() -> SessionManager:
+    return SessionManager(session_ttl_seconds=60)