fix(mail): Align gt mail send --priority with bd mail send (gt-kspu)

Changed --priority flag from string values (low/normal/high/urgent) to
numeric values (0-4) to match bd mail send interface:
- 0 = urgent/critical
- 1 = high
- 2 = normal (default)
- 3 = low
- 4 = backlog

Also added --urgent flag as shortcut for --priority 0.

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

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

internal/mail/types.go

@@ -219,6 +219,24 @@ func ParsePriority(s string) Priority {
 	}
 }
 
+// PriorityFromInt converts a beads-style integer priority to a Priority.
+// Accepts: 0=urgent, 1=high, 2=normal, 3=low, 4=backlog (treated as low).
+// Invalid values default to PriorityNormal.
+func PriorityFromInt(p int) Priority {
+	switch p {
+	case 0:
+		return PriorityUrgent
+	case 1:
+		return PriorityHigh
+	case 2:
+		return PriorityNormal
+	case 3, 4:
+		return PriorityLow
+	default:
+		return PriorityNormal
+	}
+}
+
 // ParseMessageType parses a message type string, returning TypeNotification for invalid values.
 func ParseMessageType(s string) MessageType {
 	switch MessageType(s) {