Fix provider-specific goals feature overrides

[Rat0323]

What changed

• Make the Codex goals checkbox reflect the effective provider configuration, including the common configuration.
• Allow each provider to save an explicit goals = true or goals = false override instead of disabling the checkbox when common configuration is present.
• Preserve provider-specific goals overrides during save normalization and runtime common-config merging.
• Keep the configuration preview consistent when [features] exists in both provider and common configuration.

Why

The manager previously read the checkbox only from the provider-local TOML. Save normalization could remove a duplicated features.goals entry from the provider because it came from the common configuration, so reopening the editor showed an unchecked box even when the effective configuration enabled the feature. The old merge path also did not reliably preserve an explicit provider override over the common value.

Validation

• npm run test
• npm run check
• npm run vite:build
• cargo fmt
• cargo test -p codex-plus-core merge_common_config
• cargo test -p codex-plus-manager normalize_settings_before_save
• Release EXE build completed successfully.