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/src/config/mod.rs

@@ -16,6 +16,7 @@ use crate::config::types::Notifications;
 use crate::config::types::OtelConfig;
 use crate::config::types::OtelConfigToml;
 use crate::config::types::OtelExporterKind;
+use crate::config::types::PluginConfig;
 use crate::config::types::SandboxWorkspaceWrite;
 use crate::config::types::ShellEnvironmentPolicy;
 use crate::config::types::ShellEnvironmentPolicyToml;
@@ -1211,6 +1212,10 @@ pub struct ConfigToml {
     /// User-level skill config entries keyed by SKILL.md path.
     pub skills: Option<SkillsConfig>,
 
+    /// User-level plugin config entries keyed by plugin name.
+    #[serde(default)]
+    pub plugins: HashMap<String, PluginConfig>,
+
     /// Centralized feature flags (new). Prefer this over individual toggles.
     #[serde(default)]
     // Injects known feature keys into the schema and forbids unknown keys.