Refactor mail-flag to use polymorphic variants and move wire modules
- Convert Flag_color to polymorphic variants (`Red, `Orange, etc.)
- Remove Imap_wire module from mail-flag, merge into ocaml-imap Flag
- Flag.system now uses polymorphic variants
- Flag.Keyword now wraps Mail_flag.Keyword.t directly
- Add flags_of_keywords/keywords_of_flags batch conversions
- Remove Jmap_wire module from mail-flag, merge into ocaml-jmap
- Add role_of_special_use/special_use_of_role to Mail_mailbox
- Add keywords_to_assoc/keywords_of_assoc to Mail_email
- Create toplevel mail_flag.mli with module aliases and full API docs
- Update all IMAP library code and tests for polymorphic variant types
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>