chore: add production deployment config and upgrade path requirements

- Add docker-compose.yml and docker-compose.example.yml for production deployment
- Add .env.example with all required environment variables
- Update architect agent with upgrade path requirements
- Update developer agent with migration best practices
- Add Phase 3 design documents (v0.3.0)
- Add ADR-0006 for participant state management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,27 @@
+# Sneaky Klaus Environment Configuration
+#
+# Copy this file to .env and customize for your environment.
+# NEVER commit .env to version control!
+
+# Required: Secret key for session signing and CSRF protection
+# Generate with: python -c "import secrets; print(secrets.token_hex(32))"
+SECRET_KEY=your-secret-key-here
+
+# Required for production: Resend API key for sending emails
+# Get your API key at https://resend.com
+# Leave empty for development mode (emails logged to stdout)
+RESEND_API_KEY=
+
+# Email sender address (must be from a verified domain in Resend)
+EMAIL_FROM=noreply@example.com
+
+# Public URL of your application (used in email links)
+# Include protocol, no trailing slash
+APP_URL=https://secretsanta.example.com
+
+# Environment mode: 'production' or 'development'
+# In development mode:
+#   - Emails are logged to stdout instead of sent
+#   - Debug mode is enabled
+#   - Session cookies don't require HTTPS
+FLASK_ENV=production