OpenCode sibling worktrees outside /tmp

Always give each agent its own sibling OpenCode workspace and bookmark, based on tracked main unless intentionally depending on another agent; push only that agent’s bookmark. Shared workspaces, another agent’s bookmark, or /tmp/opencode look convenient but cause collisions or inaccessible files. In colocated jj repos, verify the intended base before editing and wire GIT_DIR/GIT_WORK_TREE only when required.