fix: surface docker's error when it rejects a .deva mount

[lroolle]

Independent of the feature stack -- branched off main, mergeable on its own.

A .deva file with a mount docker rejects made deva.sh die silently: "Creating
persistent container: " and nothing else. No container, no error.

deva.sh, persistent-container path (the default; ephemeral is opt-in):

error_output=$(docker "${DOCKER_ARGS[@]}" tail -f /dev/null 2>&1)
docker_exit=$?              # under set -euo pipefail: NEVER reached
if [ $docker_exit -ne 0 ]; then ...   # dead code on failure

Under set -euo pipefail a failing command-substitution assignment aborts the
script AT the assignment, so the entire author-written error block never runs.
It was unreachable on the one path it exists for. Bind sources resolve on the
HOST fs (deva talks to the host daemon), so any VOLUME= docker rejects lands
exactly here.

Keeping the assignment left of || suspends set -e and captures the real exit;
the existing handler then prints docker's own stderr.

Test plan

• repro with .deva VOLUME=/tmp:/mnt/bad:rx
  before: exit 125, output ends at "Creating persistent container", silent
  after: exit 1, "docker: Error response from daemon: invalid mode: rx"
• success path unaffected (container created and attached)
• full local suite green (9/9), shellcheck --severity=error clean

Closes #484

[claude-yolo]

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[Copilot]

Pull request overview

Fixes a silent failure in deva.sh’s persistent-container creation path by ensuring Docker errors (e.g., invalid mount options from a .deva file) are captured and printed even under set -euo pipefail.

Changes:

• Make the docker ... command-substitution assignment non-fatal under set -e by placing it in an || list and capturing the real exit status.
• Preserve and surface Docker’s stderr via the existing error handling block (instead of aborting before it runs).
• Add an in-line comment documenting why the || docker_exit=$? pattern is required here.

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