Feature flag management for gradual rollout of new functionality.

This namespace provides pure functions for checking feature flags based on
environment variables and configuration.

The environment map is always a parameter. Each of these functions used to
have a convenience arity that called `(System/getenv)` itself, in a
namespace whose own docstring promised no side effects — so the promise and
the code disagreed, and every caller of a 1-arity was reading ambient
process state from the functional core (BOU-302). The convenience lives in
`wagoe.platform.shell.feature-flags`, which is a shell namespace and may
read the environment.

Design Principles:
- Pure functions: No side effects, configuration passed as parameter
- Explicit defaults: Clear behavior when flag not set
- Type-safe: Boolean coercion with validation
- Backward compatible: Defaults to false for new features