refactor(tools): extract shared logic for internal channels and tool definitions

- Add constants package with IsInternalChannel helper to centralize internal channel checks across agent, channels, and heartbeat services
- Add ToProviderDefs method to ToolRegistry to consolidate tool definition conversion logic used in agent loop and tool loop
- Refactor SubagentTool.Execute to use RunToolLoop for consistent tool execution with iteration tracking
- Remove duplicate inline map definitions and type assertion code throughout codebase

pkg/constants/channels.go

@@ -0,0 +1,15 @@
+// Package constants provides shared constants across the codebase.
+package constants
+
+// 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,
+}
+
+// IsInternalChannel returns true if the channel is an internal channel.
+func IsInternalChannel(channel string) bool {
+	return InternalChannels[channel]
+}