refactor: replace bool map with set-style map for internal channels

Use map[string]struct{} and comma-ok idiom for clearer and more idiomatic membership checks.

pkg/constants/channels.go

@@ -1,15 +1,16 @@
 // Package constants provides shared constants across the codebase.
 package constants
 
-// InternalChannels defines channels that are used for internal communication
+// internalChannels defines channels that are used for internal communication
 // and should not be exposed to external users or recorded as last active channel.
-var InternalChannels = map[string]bool{
-	"cli":      true,
-	"system":   true,
-	"subagent": true,
+var internalChannels = map[string]struct{}{
+	"cli":      {},
+	"system":   {},
+	"subagent": {},
 }
 
 // IsInternalChannel returns true if the channel is an internal channel.
 func IsInternalChannel(channel string) bool {
-	return InternalChannels[channel]
+	_, ok := internalChannels[channel]
+	return ok
 }