Add authentication profiles

[muxx]

Summary

• store Redmine authentication in named profiles with a current profile
• add global --profile / REDMINE_PROFILE, auth list, auth use, and profile-aware logout
• update generated usage docs and README examples for multi-Redmine workflows

Behavior

• successful auth login stores or updates the selected profile and switches current_profile to it
• profile selection order is --profile, REDMINE_PROFILE, current_profile, default

Verification

• make check
• make build
• bin/redmine --help
• bin/redmine auth --help