Support enable/disable skill via config/api. (#9328)

In config.toml:
```
[[skills.config]]
path = "/Users/xl/.codex/skills/my_skill/SKILL.md"
enabled = false
```

API:
skills/list, skills/config/write

codex-rs/core/src/config/mod.rs

@@ -13,6 +13,7 @@ use crate::config::types::SandboxWorkspaceWrite;
 use crate::config::types::ScrollInputMode;
 use crate::config::types::ShellEnvironmentPolicy;
 use crate::config::types::ShellEnvironmentPolicyToml;
+use crate::config::types::SkillsConfig;
 use crate::config::types::Tui;
 use crate::config::types::UriBasedFileOpener;
 use crate::config_loader::ConfigLayerStack;
@@ -911,6 +912,9 @@ pub struct ConfigToml {
     /// Nested tools section for feature toggles
     pub tools: Option<ToolsToml>,
 
+    /// User-level skill config entries keyed by SKILL.md path.
+    pub skills: Option<SkillsConfig>,
+
     /// Centralized feature flags (new). Prefer this over individual toggles.
     #[serde(default)]
     // Injects known feature keys into the schema and forbids unknown keys.