Update ADK doc according to issue #1521 - 4

docs/integrations/slack.md

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+# Slack Integration
+
+The ADK Slack integration provides a `SlackRunner` to easily deploy your agents
+on Slack using [Socket Mode](https://api.slack.com/apis/connections/socket).
+
+## Prerequisites
+
+Install the ADK with Slack support:
+
+```bash
+pip install "google-adk[slack]"
+```
+
+## Slack App Configuration
+
+To use the `SlackRunner`, you need to set up a Slack App in the
+[Slack API Dashboard](https://api.slack.com/apps).
+
+### 1. Enable Socket Mode
+In your app settings, go to **Socket Mode** and toggle **Enable Socket Mode** to
+`on`.
+You will be prompted to generate an **App-Level Token** (starts with `xapp-`).
+Ensure it has the `connections:write` scope.
+
+### 2. Configure Scopes
+Navigate to **OAuth & Permissions** and add the following **Bot Token Scopes**:
+
+- `app_mentions:read`: To receive mention events.
+- `chat:write`: To send messages.
+- `im:history`: To respond in Direct Messages.
+- `groups:history` (Optional): To respond in private channels.
+- `channels:history` (Optional): To respond in public channels.
+
+### 3. Subscribe to Events
+Go to **Event Subscriptions**:
+
+- Toggle **Enable Events** to `on`.
+- Under **Subscribe to bot events**, add:
+    - `app_mention`: To respond when the bot is mentioned.
+    - `message.im`: To respond in Direct Messages.
+
+### 4. Install App to Workspace
+Install the app to your workspace to obtain the
+**Bot User OAuth Token** (starts with `xoxb-`).
+
+## Usage
+
+```python
+import asyncio
+import os
+from google.adk.runners import Runner
+from google.adk.integrations.slack import SlackRunner
+from slack_bolt.app.async_app import AsyncApp
+
+async def main():
+    # 1. Initialize your ADK Runner (with your agent)
+    # runner = Runner(agent=my_agent, session_service=my_session_service)
+
+    # 2. Initialize Slack AsyncApp with your Bot Token
+    slack_app = AsyncApp(token=os.environ["SLACK_BOT_TOKEN"])
+
+    # 3. Initialize the SlackRunner
+    slack_runner = SlackRunner(runner=runner, slack_app=slack_app)
+
+    # 4. Start the runner in Socket Mode with your App Token
+    await slack_runner.start(app_token=os.environ["SLACK_APP_TOKEN"])
+
+if __name__ == "__main__":
+    asyncio.run(main())
+```
+
+## Session Management
+
+The `SlackRunner` automatically manages conversation sessions:
+
+- **Direct Messages**: The `channel_id` is used as the session ID.
+- **Threaded Conversations**: The combination of `channel_id` and `thread_ts`
+(the timestamp of the parent message) is used as the session ID to maintain
+thread context.
+- **App Mentions**: If not in a thread, the message timestamp (`ts`) is used
+with the `channel_id` to start a new threaded session if the user replies
+in-thread.