fix: agent Dockerfile package structure and Dead container crash

- Dockerfile: COPY to /app/agent/ and use agent.main:app for proper
  package imports
- docker_ops: use low-level API in get_health() to avoid NotFound on
  containers stuck in Docker Dead state
- Add comprehensive README with architecture, API docs, and usage

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

agent/docker_ops.py

@@ -217,8 +217,12 @@ class DockerOps:
             return {"success": False, "message": str(exc)}
 
     def get_health(self) -> dict:
-        """Return node health info: hostname and container count."""
-        containers = self.client.containers.list(all=True)
+        """Return node health info: hostname and container count.
+
+        Uses the low-level API to avoid NotFound errors from containers
+        stuck in Docker's 'Dead' state.
+        """
+        containers = self.client.api.containers(all=True)
         return {
             "status": "healthy",
             "hostname": socket.gethostname(),