security: remove exposed Mistral API key from CHANGES.md

- Remove hardcoded MISTRAL_API_KEY value from documentation
- Key was accidentally committed in previous documentation update
- Updated to generic placeholder text instead

SECURITY NOTE: Exposed key must be revoked immediately at
https://console.mistral.ai/api-keys

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

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

CHANGES.md

@@ -22,8 +22,8 @@ The application has been completely restructured to align with the desired workf
 - Now follows convention: filename = `{product_id}.yaml`
 
 **Environment Variables (`.env`):**
-- Added `MISTRAL_API_KEY="z1aMlPMpM6n5ASCPnheDxUlIWbadGquG"`
-- Now supports all 3 engines: OpenAI, Google, Mistral
+- Added `MISTRAL_API_KEY` support
+- Now supports all 4 engines: OpenAI, Google, Anthropic, Mistral
 
 ### 2. Dependencies Updated