Add unit tests for env-interpolator module #272

Description

The core/config/env-interpolator module lacks unit test coverage. This module handles environment variable interpolation in configuration values per ADR-PRO-002.

Scope

• ENV_VAR_PATTERN regex validation
• interpolateString — ${VAR} and ${VAR:-default} resolution
• interpolateEnvVars — recursive walk of objects/arrays
• lintEnvPatterns — detection of env patterns in committed config files

Acceptance Criteria

• All exported functions covered
• Edge cases: missing vars, empty defaults, special characters, nested structures
• Warning collection verified
• All tests passing