[Phase 2] Environment variable override support

[cbaugus]

Allow environment variables to override YAML config values. Syntax: ${ENV_VAR} or ${ENV_VAR:default}. Support in any string field. Useful for CI/CD (override URLs, credentials). Document override precedence: CLI args > Env vars > YAML > Defaults.

[cbaugus]

✅ Completed in Phase 2 (merged to dev branch)

All Phase 2 configuration system features implemented and merged:

• Complete YAML configuration system
• Schema validation and versioning
• Environment variable overrides
• Config hot-reload
• Auto-documentation generator

See PHASE2_PLAN.md for details.