Resolution: Cannot Reproduce
Add keys for authenticated login. Works fine with who-am-i, enable-job, disable-job, delete-job, create-job, but get-job and update-job fail claiming there is no job by that name. The very same command line with enable-job (as well as the others) works fine.
Workaround is to see if enable-job fails (which it will if there is no job by that name, which was the point of using get-job), then delete-job and create-job instead of update-job if the job already exists. Note that this workaround causes the loss of job history upon deletion.
% java -jar jenkins/_jenkins-cli.jar -s http://localhost:9090/ -i jenkins/id_rsa get-job trunk-pacman.integration.vdev-regression
No such job 'trunk-pacman.integration.vdev-regression'
java -jar jenkins-cli.jar get-job args...
Dumps the job definition XML to stdout
JOB : Name of the job
--username VAL : User name to authenticate yourself to Jenkins
--password VAL : Password for authentication. Note that passing a
password in arguments is insecure.
--password-file VAL : File that contains the password
% java -jar jenkins/_jenkins-cli.jar -s http://localhost:9090/ -i jenkins/id_rsa enable-job trunk-pacman.integration.vdev-regression
% echo $?