Context

PR #75 implements the core YAML config loader with layered merging and Pydantic validation. Two acceptance criteria from #59 were deferred to keep the PR focused:

1. Environment variable substitution — ${ENV_VAR} placeholders in config values should be expanded before validation
2. Config file discovery — load_config() should search standard locations (./, ~/.ai-company/, etc.) when no explicit path is given

Acceptance Criteria

• ${ENV_VAR} syntax in any string value is replaced with the environment variable's value before Pydantic validation
• Missing env vars raise ConfigValidationError with a clear message naming the variable
• Optional default syntax ${ENV_VAR:-default} is supported
• discover_config() searches: ./ai-company.yaml, ./config/ai-company.yaml, ~/.ai-company/config.yaml (in order)
• load_config() accepts config_path=None and falls back to discovery
• ConfigFileNotFoundError raised when no config found in any location
• Unit tests for: substitution (present/missing/default), discovery (each location, precedence, not found)

Design Spec Reference

Sections 4.3, 5.4

Notes

• Keep load_config(config_path: Path | str) signature backward-compatible — make config_path optional with None default
• Env var expansion should happen on the merged dict (after deep merge, before Pydantic validation)
• Use re.sub with os.environ.get() for substitution — no need for a templating library