docs: Document template placeholder exception

Stack templates intentionally contain placeholders for end-user customization.
Clarifies this is acceptable and expected behavior.

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

Author: TaylorHonaker

CONTRIBUTING.md

@@ -47,6 +47,12 @@ Guidelines for customizing and extending the codebase-optimizer framework.
 - No placeholder text (`TODO`, `PLACEHOLDER`, `TBD`)
 - Code blocks should have language tags
 
+### Exception: Stack Template Placeholders
+
+Stack-specific templates (`CLAUDE-nodejs.md`, `CLAUDE-python.md`, etc.) intentionally contain placeholder text like `[Your application purpose]` and `[Modular monolith | Microservices | Serverless]` since they are designed to be copied and customized by end users.
+
+This is the **only** acceptable use of placeholders in the repository. The base `CLAUDE.md.template` also contains these by design.
+
 ---
 
 ## Adding Custom Agents