Update Slack integration docs for better security clarity

[codegen-sh]

This PR addresses feedback about inaccurate language in our Slack integration documentation.

Changes Made

• Clarified message visibility: Updated language to explicitly state that Codegen only sees messages in threads it has been invited into
• Removed confusing language: Dropped the problematic phrase "has visibility into previous messages in the conversation" for direct messages
• Enhanced security clarity: Made the security boundaries more explicit for both DMs and channel messages
• Improved accuracy: Ensures documentation accurately reflects how the Slack integration actually works

Context

The previous documentation was causing confusion for users during security reviews, as it wasn't clear about the scope of message visibility. This update makes it crystal clear that:

1. For DMs: Codegen only sees messages in the direct message conversation where it has been invited
2. For channels: Codegen only sees messages in threads that it has been invited into

These changes make the documentation more security-friendly and accurate.

---

💻 View my work • 👤 Initiated by Jay Hack • 💬 Initiated in #development • About Codegen
⛔ Remove Codegen from PR • 🚫 Ban action checks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.