🔧(summary) add PostHog configuration example to summary env file

Include PostHog analytics configuration example in the summary
environment file with default disabled state. This provides developers
with clear setup guidance while maintaining privacy-first defaults.

env.d/development/summary.dist

@@ -12,3 +12,6 @@ OPENAI_API_KEY="your-openai-key"
 
 WEBHOOK_API_TOKEN="secret"
 WEBHOOK_URL="https://configure-your-url.com"
+
+POSTHOG_API_KEY="your-posthog-key"
+POSTHOG_ENABLED="False"