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/common/test_codex.rs

@@ -28,6 +28,7 @@ use crate::responses::start_mock_server;
 use crate::wait_for_event;
 
 type ConfigMutator = dyn FnOnce(&mut Config) + Send;
+type PreBuildHook = dyn FnOnce(&Path) + Send + 'static;
 
 /// A collection of different ways the model can output an apply_patch call
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -51,6 +52,7 @@ pub enum ShellModelOutput {
 pub struct TestCodexBuilder {
     config_mutators: Vec<Box<ConfigMutator>>,
     auth: CodexAuth,
+    pre_build_hooks: Vec<Box<PreBuildHook>>,
 }
 
 impl TestCodexBuilder {
@@ -74,6 +76,14 @@ impl TestCodexBuilder {
         })
     }
 
+    pub fn with_pre_build_hook<F>(mut self, hook: F) -> Self
+    where
+        F: FnOnce(&Path) + Send + 'static,
+    {
+        self.pre_build_hooks.push(Box::new(hook));
+        self
+    }
+
     pub async fn build(&mut self, server: &wiremock::MockServer) -> anyhow::Result<TestCodex> {
         let home = Arc::new(TempDir::new()?);
         self.build_with_home(server, home, None).await
@@ -137,6 +147,9 @@ impl TestCodexBuilder {
         let mut config = load_default_config_for_test(home);
         config.cwd = cwd.path().to_path_buf();
         config.model_provider = model_provider;
+        for hook in self.pre_build_hooks.drain(..) {
+            hook(home.path());
+        }
         if let Ok(cmd) = assert_cmd::Command::cargo_bin("codex") {
             config.codex_linux_sandbox_exe = Some(PathBuf::from(cmd.get_program().to_os_string()));
         }
@@ -171,6 +184,10 @@ impl TestCodex {
         self.cwd.path()
     }
 
+    pub fn codex_home_path(&self) -> &Path {
+        self.config.codex_home.as_path()
+    }
+
     pub fn workspace_path(&self, rel: impl AsRef<Path>) -> PathBuf {
         self.cwd_path().join(rel)
     }
@@ -351,5 +368,6 @@ pub fn test_codex() -> TestCodexBuilder {
     TestCodexBuilder {
         config_mutators: vec![],
         auth: CodexAuth::from_api_key("dummy"),
+        pre_build_hooks: vec![],
     }
 }