fix(completion): offer unique hostnames without instance prefix

bash (and zsh without a matcher) only accept candidates that start with
the typed text, so "fsn1-bx1<Tab>" found nothing when every candidate
was qualified as "prod/…". Hostnames that exist in only one instance
are now offered bare; names present in several instances stay
qualified. Connecting to a bare name still checks every instance for
ambiguity, so the connection behavior is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -552,6 +552,10 @@ rm -f ~/.zcompdump* && exec zsh
 ```
 
 Completions are served from the local cache — no network request on every `<Tab>`.
+With several NetBox instances, a hostname that exists in only one instance is
+completed without the instance prefix (`web01`); names that exist in several
+instances are offered as `prod/web01`, `staging/web01`. In bash, type the
+instance prefix for those; zsh and fish offer them for the hostname alone.
 The zsh script that `netssh completion zsh` prints is built for large caches:
 5000 hosts complete in well under a second. To limit the number of hostnames
 offered, set `completion.max_results`.