Fix witness session parsing in categorizeSession

Witness sessions use format gt-witness-<rig> but categorizeSession was
parsing them as gt-<rig>-<type>, incorrectly setting Rig="witness".
This caused the mayor status line to count "witness" as a third rig.

Added special case to handle gt-witness-<rig> format before the generic
rig-level parsing. Also updated tests to cover both actual and legacy formats.

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

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

internal/cmd/agents.go

@@ -99,6 +99,13 @@ func categorizeSession(name string) *AgentSession {
 		return session
 	}
 
+	// Witness sessions use different format: gt-witness-<rig>
+	if strings.HasPrefix(suffix, "witness-") {
+		session.Type = AgentWitness
+		session.Rig = strings.TrimPrefix(suffix, "witness-")
+		return session
+	}
+
 	// Rig-level agents: gt-<rig>-<type> or gt-<rig>-crew-<name>
 	parts := strings.SplitN(suffix, "-", 2)
 	if len(parts) < 2 {