feat: load from plugins (#12864)

Support loading plugins.

Plugins can now be enabled via [plugins.<name>] in config.toml. They are
loaded as first-class entities through PluginsManager, and their default
skills/ and .mcp.json contributions are integrated into the existing
skills and MCP flows.

codex-rs/core/tests/suite/mod.rs

@@ -95,6 +95,7 @@ mod pending_input;
 mod permissions_messages;
 mod personality;
 mod personality_migration;
+mod plugins;
 mod prompt_caching;
 mod quota_exceeded;
 mod read_file;