Add WebUI URL to Slack messages at the end of agent loop (#272)

## Description

This PR adds the WebUI URL link to Slack messages when the agent
finishes its turn. This makes it easier for users to access the WebUI
session from anywhere in a long Slack thread without having to scroll
back to the beginning.

## Implementation

- Added an optional `appendWebappUrl` parameter to `sendSystemMessage`
function with default value of `false`
- When `appendWebappUrl` is `true`, the function appends a markdown link
to the webapp session URL at the end of the Slack message:
`([webapp](URL))`
- Modified the agent loop to use this parameter only at the end of the
agent turn

## Changes

- Modified `packages/agent-core/src/lib/messages.ts` to add the optional
parameter and webapp URL handling
- Modified `packages/worker/src/agent/index.ts` to pass `true` for the
parameter at the end of the agent loop

## Related Issues

Fixes #270

<!-- DO NOT EDIT: System generated metadata -->
<!-- WORKER_ID:1751622968753739 -->

---

**Open in Web UI**:
https://d2c09i1k2ray87.cloudfront.net/sessions/1751622968753739

---------

Co-authored-by: remote-swe-app[bot] <123456+remote-swe-app[bot]@users.noreply.github.com>

Author: remote-swe-user

packages/agent-core/src/lib/messages.ts

@@ -8,6 +8,7 @@ import { ddb, TableName } from './aws/ddb';
 import { writeBytesToKey, getBytesFromKey } from './aws/s3';
 import { sendWebappEvent } from './events';
 import { sendMessageToSlack } from './slack';
+import { getWebappSessionUrl } from './webapp-origin';
 import { MessageItem } from '../schema';
 
 // Maximum input token count before applying middle-out strategy
@@ -305,9 +306,20 @@ const postProcessMessageContent = async (content: string) => {
   return flattenedArray;
 };
 
-export const sendSystemMessage = async (workerId: string, message: string) => {
-  await Promise.all([
-    sendWebappEvent(workerId, { type: 'message', role: 'assistant', message }),
-    sendMessageToSlack(message),
-  ]);
+export const sendSystemMessage = async (workerId: string, message: string, appendWebappUrl: boolean = false) => {
+  // Always send original message to webapp
+  await sendWebappEvent(workerId, { type: 'message', role: 'assistant', message });
+
+  // For Slack, optionally append webapp URL
+  if (appendWebappUrl) {
+    const sessionUrl = await getWebappSessionUrl(workerId);
+    if (sessionUrl) {
+      const slackMessage = `${message} ([webapp](${sessionUrl}))`;
+      await sendMessageToSlack(slackMessage);
+    } else {
+      await sendMessageToSlack(message);
+    }
+  } else {
+    await sendMessageToSlack(message);
+  }
 };

packages/worker/src/agent/index.ts

@@ -434,7 +434,8 @@ Users will primarily request software engineering assistance including bug fixes
       const responseText = finalMessage.content?.at(-1)?.text ?? finalMessage.content?.at(0)?.text ?? '';
       // remove <thinking> </thinking> part with multiline support
       const responseTextWithoutThinking = responseText.replace(/<thinking>[\s\S]*?<\/thinking>/g, '');
-      await sendSystemMessage(workerId, `${mention}${responseTextWithoutThinking}`);
+      // Pass true to appendWebappUrl parameter to add the webapp URL to the Slack message at the end of agent loop
+      await sendSystemMessage(workerId, `${mention}${responseTextWithoutThinking}`, true);
       break;
     }
   }