Add skills.roots config for explicit skill roots

[prabhata-oai]

Summary

• Add skills.roots to skills config so users can specify absolute local skill folders.
• When skills.roots is non-empty, use configured roots instead of default local roots.
• Preserve plugin-provided roots.
• Add tests for roots override behavior and config parsing.
• Regenerate core/config.schema.json to include the new config field.

Validation

• cargo test -p codex-core config_schema_matches_fixture -- --nocapture
• cargo test -p codex-core skill_roots_from_layer_stack_can_override_defaults_with_explicit_roots -- --nocapture
• cargo test -p codex-core parses_bundled_skills_config -- --nocapture

Tickets

• None

[github-actions]

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