Create developer retention insights in `insights/developer-retention`

[ccerv1]

Vision: Comprehensive retention analysis by ecosystem and cohort year

Required Visualization:

1. Retention Graph Format:
   • Mixed-panel graph style
   • X-axis: Years after developer first became active
   • Y-axis: Retention rate (percentage)
   • Multiple lines: One line per cohort year
2. Features:
   • Ecosystem selector (e.g., select Ethereum)
   • Multiple lines showing different cohort years:
     • 2021 cohort
     • 2022 cohort
     • 2023 cohort
     • 2024 cohort
     • 2025 cohort (and future years)
   • Shows retention getting better or worse over time by ecosystem
3. Metrics:
   • Calculate retention rate: % of developers from a given cohort still active after N years
   • Show drop-off rate over time
   • Allow comparison of retention curves across different ecosystems
4. Example Use Case:
   • "For Ethereum ecosystem, show retention of developers who first became active in 2021, 2022, 2023, etc."
   • Compare how retention has changed over time
   • Identify if retention is improving or declining

Implementation Notes:

• Need to identify "first active" date for each developer in an ecosystem
• Calculate what percentage remain active after 1 year, 2 years, 3 years, etc.
• Plot as cohort retention curves

Examples:

From Speedrun Ethereum analysis

Standard mixpanel retention chart. We'd want the verticals to be year and the horizontals to be months.

[linear]

OSO-1432