feat: Support loading skills from .agents/skills (#10317)

This PR adds support for loading
[skills](https://developers.openai.com/codex/skills) from
`.agents/skills/`.
- Issue: https://github.com/agentskills/agentskills/issues/15
- Motivation: When skills live on the filesystem, sharing them across
agents is awkward and often ends up requiring symlinks/duplication. A
single location under `.agents/` makes it easier to share skills.
- Loading from `.codex/skills/` will remain but will be deprecated soon.
The change only applies to the [REPO
scope](https://developers.openai.com/codex/skills#where-to-save-skills).
- Documentation will be updated before this change is live.

Testing with skills in two locations of this repo:
<img width="960" height="152" alt="image"
src="https://github.com/user-attachments/assets/28975ff9-7363-46dd-ad40-f4c7bfdb8234"
/>

When starting Codex with CWD in `$repo_root` (should only pick up at
root):
<img width="513" height="143" alt="image"
src="https://github.com/user-attachments/assets/389e1ea7-020c-481e-bda0-ce58562db59f"
/>

When starting Codex with CWD in `$repo_root/codex-rs` (should pick up at
cwd and crawl up to root):
<img width="552" height="177" alt="image"
src="https://github.com/user-attachments/assets/a5beb8de-11b4-45ed-8660-80707c77006a"
/>

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

@@ -425,7 +425,9 @@ async fn load_requirements_from_legacy_scheme(
 ///   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.
-fn project_root_markers_from_config(config: &TomlValue) -> io::Result<Option<Vec<String>>> {
+pub(crate) fn project_root_markers_from_config(
+    config: &TomlValue,
+) -> io::Result<Option<Vec<String>>> {
     let Some(table) = config.as_table() else {
         return Ok(None);
     };
@@ -454,7 +456,7 @@ fn project_root_markers_from_config(config: &TomlValue) -> io::Result<Option<Vec
     Ok(Some(markers))
 }
 
-fn default_project_root_markers() -> Vec<String> {
+pub(crate) fn default_project_root_markers() -> Vec<String> {
     DEFAULT_PROJECT_ROOT_MARKERS
         .iter()
         .map(ToString::to_string)