[sa-server][tui] keep skills available when workspace plugins are disabled, extend to TUI

[zamoshchin-openai]

Summary

• Keep plugin-backed skills available from skills/list when workspace plugin UI is disabled.
• Mirror the app-server plugin feature state into the TUI bootstrap path so disabled workspace plugin visibility is respected without suppressing Skills.
• Add focused regression coverage for the disabled-workspace case.

Why

Workspace-level plugin disabling is meant to hide plugin UI in Codex, not remove Skills from the product surface. The previous app-server filtering path dropped plugin-backed skill roots entirely, which made the Skills fallback disappear as well.

TUI bootstrap detail

codex-rs/tui/src/app.rs applies the effective plugin visibility returned by app-server bootstrap back into the local TUI config. If the server says plugins are disabled for the workspace, the TUI disables Feature::Plugins locally so later TUI behavior follows the server-side policy while Skills remain available. If that local feature update fails, the TUI logs a warning and continues startup instead of turning the bootstrap path into a hard failure.

Validation

• cargo test -p codex-app-server --test all skills_list_keeps_plugin_skills_when_workspace_codex_plugins_disabled
• cargo test -p codex-tui plugins_enabled_from_feature_list