Document local-only helper folder

lib/local/README.md

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+# Local-Only Backend Helpers
+
+This folder is for backend code that is useful in local development, local profiling,
+test support, or E2E support, but should not be treated as normal production runtime code.
+
+## Why this folder exists
+
+The production Docker image currently copies these paths:
+
+- `app`
+- `bin`
+- `config`
+- `db`
+- `public`
+- `script`
+- `config.ru`
+- `Rakefile`
+
+It does not copy `lib`, `spec`, `e2e`, or `tasks.py`.
+
+That means code placed in `lib/local/` may not end up in the production image.
+
+## What belongs here
+
+- request profiling helpers used only in local or test flows
+- support code for local verification
+- helper code used by local-only workflows that should stay outside the production runtime
+
+## What should not go here
+
+- normal application runtime code needed in production
+- code that controllers, models, jobs, or services must always have available in production
+
+## Related special folders
+
+Depending on purpose, local-only or non-production support code may also belong in:
+
+- `spec/` for test-only code
+- `e2e/` for reusable HTTP E2E helpers and scenarios
+- task-layer code such as `tasks.py` for local/CI workflows
+
+If code in this folder is referenced from production code paths, those call sites must guard
+against production usage and fail safe when the helper is absent.