feat(install): default to npm for `deno install -g` unprefixed packages (#34290)
`deno install -g @earendil-works/pi-coding-agent` previously bailed with
a "missing a prefix" error and asked the user to retype it with `npm:`, while `deno add @earendil-works/pi-coding-agent` and local `deno install @earendil-works/pi-coding-agent` have defaulted to the npm
registry since #33246. The global install path lives in `cli/tools/installer/global.rs` and was missed in that change.