Eliminate conversation history duplication in AI prompt

[Copilot]

The conversation history included the triggering message, then the task section redundantly instructed "Respond to the user's latest message," causing the same message to be referenced twice in the prompt.

Changes

• Mark latest message inline: Added ⬅ RESPOND TO THIS marker to the last message in conversation history
• Remove redundant instruction: Deleted separate "Respond to the user's latest message" line from task section

Example

Before:

for (const turn of conversationHistory) {
  userPrompt += `[${role}] ${turn.content}\n\n`;
}
userPrompt += `--- YOUR TASK ---\n`;
userPrompt += `Respond to the user's latest message. `;

After:

for (let i = 0; i < conversationHistory.length; i++) {
  const marker = (i === conversationHistory.length - 1) ? ' ⬅ RESPOND TO THIS' : '';
  userPrompt += `[${role}]${marker} ${turn.content}\n\n`;
}
userPrompt += `--- YOUR TASK ---\n`;

Reduces token usage and eliminates ambiguity about which message requires a response.

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