feat(mail): Add --all flag to gt mail clear for agent ergonomics (gt-105q3)

The command already clears all messages by default, but agents naturally
try --all when they want to clear everything. Adding it as a no-op flag
improves discoverability and natural usage patterns.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/mail.go

@@ -55,6 +55,9 @@ var (
 
 	// Announces flags
 	mailAnnouncesJSON bool
+
+	// Clear flags
+	mailClearAll bool
 )
 
 var mailCmd = &cobra.Command{
@@ -438,6 +441,9 @@ func init() {
 	// Announces flags
 	mailAnnouncesCmd.Flags().BoolVar(&mailAnnouncesJSON, "json", false, "Output as JSON")
 
+	// Clear flags
+	mailClearCmd.Flags().BoolVar(&mailClearAll, "all", false, "Clear all messages (default behavior)")
+
 	// Add subcommands
 	mailCmd.AddCommand(mailSendCmd)
 	mailCmd.AddCommand(mailInboxCmd)