Improve message length handling for Slack messages

[blinkagent]

Problem

We need better handling of message length limits when sending Slack messages. Previously, this was part of the Slack prompt and worked well to prevent messages from being truncated or split unexpectedly.

Proposed Solution

Reintroduce explicit message length handling guidance in the Slack prompt to ensure Blink:

• Respects Slack's message length limits
• Splits long messages appropriately
• Uses text snippets for longer content when needed
• Maintains context and readability across message boundaries

Context

This functionality existed before and worked effectively. We should restore it to improve the user experience when Blink sends longer responses via Slack.