fix: install names the targets it found

[vshulcz]

before: $ deja install
        deja: install needs target
after:  $ deja install                 # agents present
        deja: install needs a target — found here: claude-code, codex (or --all, --auto)
        $ deja install                 # clean machine
        deja: install needs a target — no agent config found here; `deja help` lists every target deja knows

deja help has the full list and the README's examples use --all/--auto, but the error named none of it — on the first command a new user types. files, restore and blame all print the shape they want; this one can also name what is actually on the machine. Closes #830.