core: trace executor skill discovery (#30318)

## Why

Make it easier to measure the performance of different parts of skill
loading.

## What

- Add spans for step-context capture, world-state construction, executor
catalog snapshot/root loading, and environment skill loading.
- Record the discovered environment skill count.
- Trace outbound exec-server requests with client kind and RPC method
fields.
- Update trace propagation tests to assert that requests keep the parent
trace id while creating their own child span.

codex-rs/core-skills/src/loader/environment.rs

@@ -143,6 +143,12 @@ pub struct EnvironmentSkillLoadOutcome {
 }
 
 /// Discovers skills without converting environment-owned paths to host paths.
+#[tracing::instrument(
+    name = "skills.environment.load",
+    level = "info",
+    skip_all,
+    fields(skill_count = tracing::field::Empty)
+)]
 pub async fn load_environment_skills_from_root(
     file_system: &dyn ExecutorFileSystem,
     root: &PathUri,
@@ -235,6 +241,7 @@ pub async fn load_environment_skills_from_root(
             warnings: vec![format!("failed to walk skills root {root}: {error:#}")],
         },
     };
+    tracing::Span::current().record("skill_count", discovery.skills.len());
     outcome.warnings.extend(discovery.warnings);
     if discovery.skills.is_empty() {
         return outcome;