Build pattern recognition across retrospectives

[romahawk]

Part of #60

What

Analyze multiple retrospectives to surface recurring themes, friction patterns, and momentum trends.

Tasks

• Create lib/ai/patterns.ts with analyzeRetroPatterns(weeklyReviews)
• Identify top-3 recurring friction themes across last 4+ weeks
• Identify top-3 momentum wins
• Expose patterns as a structured object: { frictionThemes, momentumWins, trendDirection }
• Requires at least 4 completed retrospectives (return null if fewer)

AC

• Correctly groups similar friction themes from multiple weeks
• Returns null gracefully when history is insufficient