bug: defaults merge replaces entire struct instead of field-wise

[EffortlessSteven]

Problem

In config_loader.rs:123-126, when merging overlay configs, the entire Defaults struct is replaced if any field differs. This means:

1. Parent config sets defaults.base = "main", defaults.scope = "added"
2. Child config sets defaults.fail_on = "error"
3. Result: only fail_on is set — base and scope are lost

Fix

Implement field-wise merge for Defaults:

• Option<T> fields: child overrides parent only if Some
• Non-option fields: child overrides parent only if non-default

Source

PR #5 review — Codex P1 (comment #2).

[EffortlessSteven]

Fixed in PR #14