fix(chat): return empty slice instead of nil from DetectAgents

When no agents are installed (e.g., CI runners), DetectAgents returned
nil which caused TestDetectAgents_Structure to fail with NotNil assert.

Author: qasim-nylas

internal/chat/agent.go

@@ -30,7 +30,7 @@ type Agent struct {
 // DetectAgents scans the system for installed AI agents.
 // It checks for claude, codex, and ollama in $PATH.
 func DetectAgents() []Agent {
-	var agents []Agent
+	agents := []Agent{}
 
 	checks := []struct {
 		name        AgentType