Remove redundant prompt instruction in conversation history flow

[Copilot]

The AI prompt instructed "Respond to the user's latest message" while that message was already included in the conversation history, creating token waste and potential confusion.

Changes:

• Updated prompt instruction from "Respond to the user's latest message" to "Continue the conversation based on the history above"

The conversation history builder (lines 139-169) already includes all messages in chronological order:

1. Issue body with title
2. All comments (user and bot) in sequence

The new instruction correctly references this complete history without implying the latest message is separate.

# Before
userPrompt += `--- YOUR TASK ---\n`;
userPrompt += `Respond to the user's latest message. `;

# After  
userPrompt += `--- YOUR TASK ---\n`;
userPrompt += `Continue the conversation based on the history above. `;

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