docs(cli): clarify migrate requires live destination access

[tymrtn]

Summary

• Clarifies envelope migrate help: it requires simultaneous source and destination IMAP access.
• Points staged provider handoffs to backup export, backup verify, and later backup restore.
• Adds a CLI help regression test so this distinction stays visible.

Test Plan

• cargo fmt --check
• cargo test -p envelope-email migrate_help_mentions_simultaneous_access_and_backup_restore -- --nocapture
• cargo test -p envelope-email help -- --nocapture

Safety

• No live mailbox export, restore, or migration was run.