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

web/frontend/src/components/config/config-page.tsx

@@ -208,6 +208,7 @@ export function ConfigPage() {
             defaults: {
               workspace,
               restrict_to_workspace: form.restrictToWorkspace,
+              split_on_marker: form.splitOnMarker,
               tool_feedback: {
                 enabled: form.toolFeedbackEnabled,
                 max_args_length: toolFeedbackMaxArgsLength,

web/frontend/src/components/config/config-sections.tsx

@@ -95,6 +95,16 @@ export function AgentDefaultsSection({
         }
       />
 
+      <SwitchCardField
+        label={t("pages.config.split_on_marker")}
+        hint={t("pages.config.split_on_marker_hint")}
+        layout="setting-row"
+        checked={form.splitOnMarker}
+        onCheckedChange={(checked) =>
+          onFieldChange("splitOnMarker", checked)
+        }
+      />
+
       <SwitchCardField
         label={t("pages.config.tool_feedback_enabled")}
         hint={t("pages.config.tool_feedback_enabled_hint")}

web/frontend/src/components/config/form-model.ts

@@ -3,6 +3,7 @@ export type JsonRecord = Record<string, unknown>
 export interface CoreConfigForm {
   workspace: string
   restrictToWorkspace: boolean
+  splitOnMarker: boolean
   toolFeedbackEnabled: boolean
   toolFeedbackMaxArgsLength: string
   execEnabled: boolean
@@ -65,6 +66,7 @@ export const DM_SCOPE_OPTIONS = [
 export const EMPTY_FORM: CoreConfigForm = {
   workspace: "",
   restrictToWorkspace: true,
+  splitOnMarker: false,
   toolFeedbackEnabled: true,
   toolFeedbackMaxArgsLength: "300",
   execEnabled: true,
@@ -136,6 +138,10 @@ export function buildFormFromConfig(config: unknown): CoreConfigForm {
       defaults.restrict_to_workspace === undefined
         ? EMPTY_FORM.restrictToWorkspace
         : asBool(defaults.restrict_to_workspace),
+    splitOnMarker:
+      defaults.split_on_marker === undefined
+        ? EMPTY_FORM.splitOnMarker
+        : asBool(defaults.split_on_marker),
     toolFeedbackEnabled:
       toolFeedback.enabled === undefined
         ? EMPTY_FORM.toolFeedbackEnabled