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/project_root_markers.rs

@@ -0,0 +1,50 @@
+use std::io;
+
+use toml::Value as TomlValue;
+
+const DEFAULT_PROJECT_ROOT_MARKERS: &[&str] = &[".git"];
+
+/// Reads `project_root_markers` from a merged `config.toml` [toml::Value].
+///
+/// Invariants:
+/// - If `project_root_markers` is not specified, returns `Ok(None)`.
+/// - If `project_root_markers` is specified, returns `Ok(Some(markers))` where
+///   `markers` is a `Vec<String>` (including `Ok(Some(Vec::new()))` for an
+///   empty array, which indicates that root detection should be disabled).
+/// - Returns an error if `project_root_markers` is specified but is not an
+///   array of strings.
+pub fn project_root_markers_from_config(config: &TomlValue) -> io::Result<Option<Vec<String>>> {
+    let Some(table) = config.as_table() else {
+        return Ok(None);
+    };
+    let Some(markers_value) = table.get("project_root_markers") else {
+        return Ok(None);
+    };
+    let TomlValue::Array(entries) = markers_value else {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            "project_root_markers must be an array of strings",
+        ));
+    };
+    if entries.is_empty() {
+        return Ok(Some(Vec::new()));
+    }
+    let mut markers = Vec::new();
+    for entry in entries {
+        let Some(marker) = entry.as_str() else {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "project_root_markers must be an array of strings",
+            ));
+        };
+        markers.push(marker.to_string());
+    }
+    Ok(Some(markers))
+}
+
+pub fn default_project_root_markers() -> Vec<String> {
+    DEFAULT_PROJECT_ROOT_MARKERS
+        .iter()
+        .map(ToString::to_string)
+        .collect()
+}