Add public skills + improve repo skill discovery and error UX (#8098)

1. Adds SkillScope::Public end-to-end (core + protocol) and loads skills
from the public cache directory
2. Improves repo skill discovery by searching upward for the nearest
.codex/skills within a git repo
3. Deduplicates skills by name with deterministic ordering to avoid
duplicates across sources
4. Fixes garbled “Skill errors” overlay rendering by preventing pending
history lines from being injected during the modal
5. Updates the project docs “Skills” intro wording to avoid hardcoded
paths

codex-rs/core/tests/suite/skills.rs

@@ -27,6 +27,20 @@ fn write_skill(home: &Path, name: &str, description: &str, body: &str) -> std::p
     path
 }
 
+fn write_public_skill(
+    home: &Path,
+    name: &str,
+    description: &str,
+    body: &str,
+) -> std::path::PathBuf {
+    let skill_dir = home.join("skills").join(".public").join(name);
+    fs::create_dir_all(&skill_dir).unwrap();
+    let contents = format!("---\nname: {name}\ndescription: {description}\n---\n\n{body}\n");
+    let path = skill_dir.join("SKILL.md");
+    fs::write(&path, contents).unwrap();
+    path
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn user_turn_includes_skill_instructions() -> Result<()> {
     skip_if_no_network!(Ok(()));
@@ -115,7 +129,10 @@ async fn skill_load_errors_surface_in_session_configured() -> Result<()> {
     let test = builder.build(&server).await?;
 
     test.codex
-        .submit(Op::ListSkills { cwds: Vec::new() })
+        .submit(Op::ListSkills {
+            cwds: Vec::new(),
+            force_reload: false,
+        })
         .await?;
     let response =
         core_test_support::wait_for_event_match(test.codex.as_ref(), |event| match event {
@@ -133,8 +150,13 @@ async fn skill_load_errors_surface_in_session_configured() -> Result<()> {
         .unwrap_or_default();
 
     assert!(
-        skills.is_empty(),
-        "expected no skills loaded, got {skills:?}"
+        skills.iter().all(|skill| {
+            !skill
+                .path
+                .to_string_lossy()
+                .ends_with("skills/broken/SKILL.md")
+        }),
+        "expected broken skill not loaded, got {skills:?}"
     );
     assert_eq!(errors.len(), 1, "expected one load error");
     let error_path = errors[0].path.to_string_lossy();
@@ -145,3 +167,52 @@ async fn skill_load_errors_surface_in_session_configured() -> Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn list_skills_includes_public_cache_entries() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut builder = test_codex()
+        .with_config(|cfg| {
+            cfg.features.enable(Feature::Skills);
+        })
+        .with_pre_build_hook(|home| {
+            write_public_skill(home, "public-demo", "public skill", "public body");
+        });
+    let test = builder.build(&server).await?;
+
+    test.codex
+        .submit(Op::ListSkills {
+            cwds: Vec::new(),
+            force_reload: true,
+        })
+        .await?;
+    let response =
+        core_test_support::wait_for_event_match(test.codex.as_ref(), |event| match event {
+            codex_core::protocol::EventMsg::ListSkillsResponse(response) => Some(response.clone()),
+            _ => None,
+        })
+        .await;
+
+    let cwd = test.cwd_path();
+    let (skills, _errors) = response
+        .skills
+        .iter()
+        .find(|entry| entry.cwd.as_path() == cwd)
+        .map(|entry| (entry.skills.clone(), entry.errors.clone()))
+        .unwrap_or_default();
+
+    let skill = skills
+        .iter()
+        .find(|skill| skill.name == "public-demo")
+        .expect("expected public skill to be present");
+    assert_eq!(skill.scope, codex_protocol::protocol::SkillScope::Public);
+    let path_str = skill.path.to_string_lossy().replace('\\', "/");
+    assert!(
+        path_str.ends_with("/skills/.public/public-demo/SKILL.md"),
+        "unexpected skill path: {path_str}"
+    );
+
+    Ok(())
+}