fix: add USER=agent to crontab env (unbound variable in cron)

env.sh references $USER which is not set in cron environment.
With set -u (pipefail), this causes env.sh to exit before setting
DISINTO_LOG_DIR, resulting in log writes to the read-only mount.

Root cause of silent cron failures since containerized setup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/agents/entrypoint.sh

@@ -18,7 +18,8 @@ log() {
 
 # Build crontab from project TOMLs and install for the agent user.
 install_project_crons() {
-  local cron_lines="DISINTO_CONTAINER=1"
+  local cron_lines="DISINTO_CONTAINER=1
+USER=agent"
   for toml in "${DISINTO_DIR}"/projects/*.toml; do
     [ -f "$toml" ] || continue
     local pname