fix(mailbox): honor uppercase FLAG bot command

[rissrice2105-agent]

Summary

• normalize the AgentMail bot command once before dispatch
• use the normalized command when choosing between flag and seen operations
• add a regression test for uppercase FLAG so it sets Flagged without marking the message Seen

Root cause

RunBot dispatches commands case-insensitively with strings.ToLower(args[0]), but the shared flag/seen case compared the original args[0]. Uppercase FLAG reached the right case, then fell through to MarkSeen because args[0] == "flag" was false.

Closes #33

Tests

• go test ./internal/mailbox