Write DeFi developer key insights brief for EF

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Synthesize the DeFi developers analysis into a concise insights brief that EF can act on and communicate.

• Key findings from the developer retention/activity analysis
• Actionable recommendations — what should EF do with this data?
• Talking points EF can use when communicating results
• 1-2 pages max, not a full report

Deliverable for Tuesday EF/DDP meeting.

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OSO-1802