Add optional PRESERVE_MEMORY_BLOCKS configuration

Provides a commented example of the new memory preservation setting for the
environment configuration.

.env.example

···
 # AUTOMATION_DESCRIPTION="refuses to open pod bay doors"
 # DISCLOSURE_URL="example.com/bot-policy"
 # RESPONSIBLE_PARTY_BSKY="DID:... or example.bsky.app, no @symbol"
+# PRESERVE_MEMORY_BLOCKS=true