cli: UX review small fixes (unregistered key, issue create flags, plurals, readme, mirror time) #268

Smaller CLI findings from the UX review.

• Unregistered key (internal/sshd/sshd.go:332): prints a literal <host> and tells everyone to register, including someone on a new laptop with an existing account. Print the fingerprint and the real host, and offer both paths: add this key at https://<host>/settings#keys, or ssh git@<host> register ....
• issue create takes only --title --body --file --format; labels, milestone and assignee each need another call. The web form takes labels at create. Add --label (repeatable), --milestone, --assignee, and a help line saying $EDITOR opens when neither --body nor --file is given.
• mr show labels plural lists in the singular (commit: over 7 rows, check: over 2). Use commits (7):, checks (2):.
• No command prints a repository's README. Add repo readme <owner/name> [--ref] using the web page's file order.
• repo show prints the mirror time with milliseconds (2026-09-24T15:31:50.839Z); truncate to the second like the other rows.