Inject SKILL.md when it's explicitly mentioned. (#7763)

1. Skills load once in core at session start; the cached outcome is
reused across core and surfaced to TUI via SessionConfigured.
2. TUI detects explicit skill selections, and core injects the matching
SKILL.md content into the turn when a selected skill is present.

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

@@ -0,0 +1,136 @@
+#![cfg(not(target_os = "windows"))]
+#![allow(clippy::unwrap_used, clippy::expect_used)]
+
+use anyhow::Result;
+use codex_core::features::Feature;
+use codex_core::protocol::AskForApproval;
+use codex_core::protocol::Op;
+use codex_core::protocol::SandboxPolicy;
+use codex_core::protocol::SkillLoadOutcomeInfo;
+use codex_protocol::user_input::UserInput;
+use core_test_support::responses::ev_assistant_message;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::mount_sse_once;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::test_codex;
+use std::fs;
+use std::path::Path;
+
+fn write_skill(home: &Path, name: &str, description: &str, body: &str) -> std::path::PathBuf {
+    let skill_dir = home.join("skills").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(()));
+
+    let server = start_mock_server().await;
+    let skill_body = "skill body";
+    let mut builder = test_codex()
+        .with_config(|cfg| {
+            cfg.features.enable(Feature::Skills);
+        })
+        .with_pre_build_hook(|home| {
+            write_skill(home, "demo", "demo skill", skill_body);
+        });
+    let test = builder.build(&server).await?;
+
+    let skill_path = test.codex_home_path().join("skills/demo/SKILL.md");
+    let skill_path = std::fs::canonicalize(skill_path)?;
+
+    let mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-1"),
+        ]),
+    )
+    .await;
+
+    let session_model = test.session_configured.model.clone();
+    test.codex
+        .submit(Op::UserTurn {
+            items: vec![
+                UserInput::Text {
+                    text: "please use $demo".to_string(),
+                },
+                UserInput::Skill {
+                    name: "demo".to_string(),
+                    path: skill_path.clone(),
+                },
+            ],
+            final_output_json_schema: None,
+            cwd: test.cwd_path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: session_model,
+            effort: None,
+            summary: codex_protocol::config_types::ReasoningSummary::Auto,
+        })
+        .await?;
+
+    core_test_support::wait_for_event(test.codex.as_ref(), |event| {
+        matches!(event, codex_core::protocol::EventMsg::TaskComplete(_))
+    })
+    .await;
+
+    let request = mock.single_request();
+    let user_texts = request.message_input_texts("user");
+    let skill_path_str = skill_path.to_string_lossy();
+    assert!(
+        user_texts.iter().any(|text| {
+            text.contains("<skill>\n<name>demo</name>")
+                && text.contains("<path>")
+                && text.contains(skill_body)
+                && text.contains(skill_path_str.as_ref())
+        }),
+        "expected skill instructions in user input, got {user_texts:?}"
+    );
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn skill_load_errors_surface_in_session_configured() -> 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| {
+            let skill_dir = home.join("skills").join("broken");
+            fs::create_dir_all(&skill_dir).unwrap();
+            fs::write(skill_dir.join("SKILL.md"), "not yaml").unwrap();
+        });
+    let test = builder.build(&server).await?;
+
+    let SkillLoadOutcomeInfo { skills, errors } = test
+        .session_configured
+        .skill_load_outcome
+        .as_ref()
+        .expect("skill outcome present");
+
+    assert!(
+        skills.is_empty(),
+        "expected no skills loaded, got {skills:?}"
+    );
+    assert_eq!(errors.len(), 1, "expected one load error");
+    let error_path = errors[0].path.to_string_lossy();
+    assert!(
+        error_path.ends_with("skills/broken/SKILL.md"),
+        "unexpected error path: {error_path}"
+    );
+
+    Ok(())
+}