Update Slack integration docs for better security clarity

- Clarify that Codegen only sees messages in threads it's invited to
- Remove confusing language about 'visibility into previous messages'
- Make security boundaries more explicit for both DMs and channels
- Address user feedback about inaccurate documentation

Author: codegen-sh[bot]

docs/integrations/slack.mdx

@@ -75,11 +75,12 @@ Codegen only responds when tagged or messaged directly. Use these approaches to
 - **Direct Messages:**
 
   - Responds to any DM sent to the Codegen integration for Slack
-  - Has visibility into previous messages in the conversation (not just threaded replies)
+  - Codegen only sees messages in the direct message conversation where it has been invited
   - This provides a more natural conversation experience as many users don't thread messages in DMs
 
 - **Channel Messages:**
   - Responds to any message that @mentions Codegen in channels where the integration for Slack is installed
+  - Codegen only sees messages in threads that it has been invited into
   - Only has visibility into the local context of the thread/conversation
   - Sending subsequent messages within a thread routes to the same agent (tag `@codegen` to trigger)
   - New messages to `@codegen` in an active thread will interrupt the agent if it's currently working