feat(channels): support multi-message sending via split marker (#2008)

* Add multi-message sending via split marker

* Add marker and length split integration tests

Tests that SplitByMarker and SplitMessage work together correctly, and
that code block boundaries are preserved during marker splitting.

* Simplify message chunking logic in channel worker

Extract splitByLength helper function and remove goto-based control
flow.
The logic now flows more naturally - try marker splitting first, then
fall
back to length-based splitting.

* Update multi-message output instructions in agent context

* Add split_on_marker to config defaults

* Add split_on_marker config option

* Rename 'Multi-Message Sending' setting to 'Chatty Mode'

* Add SplitOnMarker config option

pkg/agent/context.go

@@ -26,6 +26,7 @@ type ContextBuilder struct {
 	memory             *MemoryStore
 	toolDiscoveryBM25  bool
 	toolDiscoveryRegex bool
+	splitOnMarker      bool
 
 	// Cache for system prompt to avoid rebuilding on every call.
 	// This fixes issue #607: repeated reprocessing of the entire context.
@@ -52,6 +53,11 @@ func (cb *ContextBuilder) WithToolDiscovery(useBM25, useRegex bool) *ContextBuil
 	return cb
 }
 
+func (cb *ContextBuilder) WithSplitOnMarker(enabled bool) *ContextBuilder {
+	cb.splitOnMarker = enabled
+	return cb
+}
+
 func getGlobalConfigDir() string {
 	if home := os.Getenv(config.EnvHome); home != "" {
 		return home
@@ -157,6 +163,14 @@ The following skills extend your capabilities. To use a skill, read its SKILL.md
 		parts = append(parts, "# Memory\n\n"+memoryContext)
 	}
 
+	// Multi-Message Sending (if enabled)
+	if cb.splitOnMarker {
+		parts = append(parts, `# MULTI-MESSAGE OUTPUT
+You MUST frequently use <|[SPLIT]|> to break your responses into multiple short messages. NEVER output a single long wall of text. Actively split distinct concepts or parts. Example: Message part 1<|[SPLIT]|>Message part 2<|[SPLIT]|>Message part 3
+
+Each part separated by the marker will be sent as an independent message.`)
+	}
+
 	// Join with "---" separator
 	return strings.Join(parts, "\n\n---\n\n")
 }

pkg/agent/instance.go

@@ -103,10 +103,12 @@ func NewAgentInstance(
 	sessions := initSessionStore(sessionsDir)
 
 	mcpDiscoveryActive := cfg.Tools.MCP.Enabled && cfg.Tools.MCP.Discovery.Enabled
-	contextBuilder := NewContextBuilder(workspace).WithToolDiscovery(
-		mcpDiscoveryActive && cfg.Tools.MCP.Discovery.UseBM25,
-		mcpDiscoveryActive && cfg.Tools.MCP.Discovery.UseRegex,
-	)
+	contextBuilder := NewContextBuilder(workspace).
+		WithToolDiscovery(
+			mcpDiscoveryActive && cfg.Tools.MCP.Discovery.UseBM25,
+			mcpDiscoveryActive && cfg.Tools.MCP.Discovery.UseRegex,
+		).
+		WithSplitOnMarker(cfg.Agents.Defaults.SplitOnMarker)
 
 	agentID := routing.DefaultAgentID
 	agentName := ""