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>

docker-compose.yml

@@ -22,7 +22,12 @@ services:
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - ./data:/data
-      - ${STACKS_HOST_DIR:-./data/stacks}:/opt/stacks
+      # MUST resolve to the same path as STACKS_DIR above: compose runs inside
+      # this container and resolves a stack's relative bind mounts (./config)
+      # against the container path, which the daemon then creates at that same
+      # path on the host. A different host path means the stacks' data lands
+      # somewhere StackPilot cannot see (file browser, editor).
+      - ${STACKS_HOST_DIR:-/opt/stacks}:/opt/stacks
       - /proc:/host_proc:ro
       # Host devices for GPU/device detection + passthrough (USB/TTY/DRI).
       # Read-only; remove if you don't need GPU/device features.