Default STACKS_HOST_DIR to /opt/stacks so host and container paths match

The shipped default (./data/stacks) guarantees the mismatch that hid stack data
from the file browser, the editor and (before 0.40.0) from backups: compose
resolves ./config against the container path, so the daemon creates the data
directories at /opt/stacks/<stack>/... on the host regardless of where
STACKS_HOST_DIR points. Same change for the agent, plus the reasoning in
.env.example and the README config table. Images unchanged (0.40.1).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

agent/docker-compose.yml

@@ -15,7 +15,10 @@ services:
       - HOST_PROC_PATH=/host_proc
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
-      - ${STACKS_HOST_DIR:-./data/stacks}:/opt/stacks
+      # Same rule as the central app: this must resolve to the same path as
+      # STACKS_DIR, or the stacks' relative bind mounts (./config) end up at the
+      # container path on the host, where the agent cannot see them.
+      - ${STACKS_HOST_DIR:-/opt/stacks}:/opt/stacks
       - /proc:/host_proc:ro
       # Read-only host devices for status/detection parity with the main host.
       - /dev:/dev:ro