3.1.0

AI generated. Reviewed by Johnny.

This release adds jd agent-setup, which has your agent write the config
for you.

Features

• jd agent-setup prints a prompt for your agent. Paste it in, and the
  agent finds your systems on disk, confirms each root with you, and
  writes ~/.jd/config.json and the source line in your shell config.
  It works before any config exists. Only the prompt goes to stdout, so
  jd agent-setup | pbcopy puts it on the clipboard.

Changes

• The "no config" error is now three lines: the file it looked for, the
  help page for a person to read, and jd agent-setup for an agent.
  Every command that needs the config says the same thing.

Closes #7.