fix: prevent duplicate opportunities based on link

[devin-ai-integration]

Prevent duplicate opportunities by checking if the link from a new Slack message already exists in any other opportunity's Slack message.

Changes:

• Add findOpportunityWithLink function to check for existing opportunities with a given link
• Modify createOpportunity to check for duplicates before creating new opportunities
• Exit gracefully (success) if duplicate found to maintain existing behavior pattern

Link to Devin run: https://app.devin.ai/sessions/70f0c979125e4468a1c03858f4d8b110

[devin-ai-integration]

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