[TASK] Refactor cmd/config.go into parser, env overlay, and defaults

[edenreich]

Summary

cmd/config.go is roughly 2,134 lines and conflates three responsibilities: YAML parsing, environment-variable overrides, and default population. The interplay between these layers is hard to reason about and makes env-override bugs hard to diagnose (precedence is implicit).

This issue tracks splitting cmd/config.go so each layer is independently testable and the precedence between layers is explicit.

Acceptance Criteria

• YAML parsing lives in its own file/package and produces a pure config struct with no side effects.
• Default population is a separate, pure function over the parsed struct.
• Environment-variable overlay is a separate, pure function with documented precedence (file < defaults < env, or whatever the chosen order is).
• A single top-level Load() composes the three steps and is the only public entry point.
• Table-driven tests cover each layer independently and the composed Load().
• The split does not change observable behavior; existing config files and env vars work as before.
• Precedence is documented in a code comment on Load() and in the user-facing config docs.