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

config/config.example.json

@@ -10,6 +10,7 @@
       "max_tool_iterations": 20,
       "summarize_message_threshold": 20,
       "summarize_token_percent": 75,
+      "split_on_marker": false,
       "tool_feedback": {
         "enabled": false,
         "max_args_length": 300
@@ -223,13 +224,8 @@
       "nickserv_password": "",
       "sasl_user": "",
       "sasl_password": "",
-      "channels": [
-        "#mychannel"
-      ],
-      "request_caps": [
-        "server-time",
-        "message-tags"
-      ],
+      "channels": ["#mychannel"],
+      "request_caps": ["server-time", "message-tags"],
       "allow_from": [],
       "group_trigger": {
         "mention_only": true
@@ -251,9 +247,7 @@
       "brave": {
         "enabled": false,
         "api_key": "YOUR_BRAVE_API_KEY",
-        "api_keys": [
-          "YOUR_BRAVE_API_KEY"
-        ],
+        "api_keys": ["YOUR_BRAVE_API_KEY"],
         "max_results": 5
       },
       "tavily": {
@@ -269,9 +263,7 @@
       "perplexity": {
         "enabled": false,
         "api_key": "pplx-xxx",
-        "api_keys": [
-          "pplx-xxx"
-        ],
+        "api_keys": ["pplx-xxx"],
         "max_results": 5
       },
       "searxng": {
@@ -320,19 +312,12 @@
         "filesystem": {
           "enabled": false,
           "command": "npx",
-          "args": [
-            "-y",
-            "@modelcontextprotocol/server-filesystem",
-            "/tmp"
-          ]
+          "args": ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
         },
         "github": {
           "enabled": false,
           "command": "npx",
-          "args": [
-            "-y",
-            "@modelcontextprotocol/server-github"
-          ],
+          "args": ["-y", "@modelcontextprotocol/server-github"],
           "env": {
             "GITHUB_PERSONAL_ACCESS_TOKEN": "YOUR_GITHUB_TOKEN"
           }
@@ -340,10 +325,7 @@
         "brave-search": {
           "enabled": false,
           "command": "npx",
-          "args": [
-            "-y",
-            "@modelcontextprotocol/server-brave-search"
-          ],
+          "args": ["-y", "@modelcontextprotocol/server-brave-search"],
           "env": {
             "BRAVE_API_KEY": "YOUR_BRAVE_API_KEY"
           }
@@ -360,10 +342,7 @@
         "slack": {
           "enabled": false,
           "command": "npx",
-          "args": [
-            "-y",
-            "@modelcontextprotocol/server-slack"
-          ],
+          "args": ["-y", "@modelcontextprotocol/server-slack"],
           "env": {
             "SLACK_BOT_TOKEN": "YOUR_SLACK_BOT_TOKEN",
             "SLACK_TEAM_ID": "YOUR_SLACK_TEAM_ID"