fix(deploy): fall back to container SHA discovery when the staging revision state file is unreadable

The staging host currently has .state/deployed-revision unreadable by the
deploy user, so the forward-only revision check failed with 'Permission
denied' before it could compare revisions. Treat an unreadable state file
like a missing one and discover the running revision from the web
container's GITHUB_SHA, in both the workflow step and the server-side
deploy/migration scripts.

deploy/run-staging-deploy.sh

@@ -47,7 +47,7 @@ flock -n 9 || {
 }
 
 current_sha="not-deployed"
-if [ -f "$state_directory/deployed-revision" ]; then
+if [ -r "$state_directory/deployed-revision" ]; then
   current_sha="$(<"$state_directory/deployed-revision")"
 else
   existing_web="$("${docker_command[@]}" ps -aq \

deploy/run-staging-migration.sh

@@ -41,7 +41,7 @@ flock -n 9 || {
 }
 
 current_sha="not-deployed"
-if [ -f "$state_directory/deployed-revision" ]; then
+if [ -r "$state_directory/deployed-revision" ]; then
   current_sha="$(<"$state_directory/deployed-revision")"
 else
   existing_web="$("${docker_command[@]}" ps -aq \