Consolidate host agent path env vars into single AGENT_FILES_ROOTDIR

Replace AGENT_KERNEL_PATH, AGENT_IMAGES_PATH, AGENT_SANDBOXES_PATH,
AGENT_SNAPSHOTS_PATH, and AGENT_TOKEN_FILE with a single
AGENT_FILES_ROOTDIR (default /var/lib/wrenn) that derives all
subdirectory paths automatically.

.env.example

@@ -10,13 +10,9 @@ CP_HOST_AGENT_ADDR=localhost:50051
 
 # Host Agent
 AGENT_LISTEN_ADDR=:50051
-AGENT_KERNEL_PATH=/var/lib/wrenn/kernels/vmlinux
-AGENT_IMAGES_PATH=/var/lib/wrenn/images
-AGENT_SANDBOXES_PATH=/var/lib/wrenn/sandboxes
-AGENT_SNAPSHOTS_PATH=/var/lib/wrenn/snapshots
+AGENT_FILES_ROOTDIR=/var/lib/wrenn
 AGENT_HOST_INTERFACE=eth0
 AGENT_CP_URL=http://localhost:8000
-AGENT_TOKEN_FILE=/var/lib/wrenn/host-token
 
 # Lago (billing — external service)
 LAGO_API_URL=http://localhost:3000