ssh agent host: accept `localhost` in startup banner regex (#314640)

ssh agent host: accept localhost in startup banner regex

The CLI's `agent host` subcommand prints its listening URL via
print_network_lines as `ws://localhost:<port>?tkn=...`, but the SSH
remote agent host startup parser was only matching `ws://127.0.0.1:`.
This caused remote agent host startup to time out after 60s.

Broaden the host portion of the regex to accept either `127.0.0.1` or
`localhost`. (Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>