Extract codex-core-skills crate (#15749)

## Summary
- move skill loading and management into codex-core-skills
- leave codex-core with the thin integration layer and shared wiring

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/config/src/skills_config.rs

@@ -0,0 +1,53 @@
+//! Skill-related configuration types shared across crates.
+
+use codex_utils_absolute_path::AbsolutePathBuf;
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+
+const fn default_enabled() -> bool {
+    true
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema)]
+#[schemars(deny_unknown_fields)]
+pub struct SkillConfig {
+    /// Path-based selector.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub path: Option<AbsolutePathBuf>,
+    /// Name-based selector.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub name: Option<String>,
+    pub enabled: bool,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq, JsonSchema)]
+#[schemars(deny_unknown_fields)]
+pub struct SkillsConfig {
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub bundled: Option<BundledSkillsConfig>,
+
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub config: Vec<SkillConfig>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema)]
+#[schemars(deny_unknown_fields)]
+pub struct BundledSkillsConfig {
+    #[serde(default = "default_enabled")]
+    pub enabled: bool,
+}
+
+impl Default for BundledSkillsConfig {
+    fn default() -> Self {
+        Self { enabled: true }
+    }
+}
+
+impl TryFrom<toml::Value> for SkillsConfig {
+    type Error = toml::de::Error;
+
+    fn try_from(value: toml::Value) -> Result<Self, Self::Error> {
+        SkillsConfig::deserialize(value)
+    }
+}