Stop spelling skills extra roots in TUI requests

[xli-oai]

Summary

• Add Default for SkillsListParams so callers can rely on the existing None default for extra roots.
• Update the TUI skills/list call sites to stop explicitly spelling per_cwd_extra_user_roots: None.
• Include the one-line PluginListParams test initializer fix needed for protocol tests on current main.

Why

This lets the client-side cleanup land before the server/API removal. The TUI still sends the same effective request today, but the call sites no longer explicitly depend on the extra-roots field.

Validation

• just fmt
• cargo test -p codex-app-server-protocol skills_list_params_serialization_uses_force_reload
• cargo check -p codex-tui
• just fix -p codex-app-server-protocol
• just fix -p codex-tui was run once, but clippy rewrote the TUI struct update back to the explicit None; the intended TUI cleanup was reapplied and validated with cargo check -p codex-tui.