Label explicit skill injection metrics (#36191)

## Why

The `codex.skill.injected` counter needs to distinguish explicitly requested
skills from implicitly invoked skills.

## What changed

Add the `invoke_type=explicit` attribute to skill injection metrics emitted for
explicit skill mentions, including both successful and failed injections.

GitOrigin-RevId: 22023f8d628be0f22b23f5376b668e8d5492c14a

codex-rs/core-skills/src/injection.rs

@@ -142,7 +142,11 @@ fn emit_skill_injected_metric(
     otel.counter(
         "codex.skill.injected",
         /*inc*/ 1,
-        &[("status", status), ("skill", skill_name_tag.as_str())],
+        &[
+            ("status", status),
+            ("skill", skill_name_tag.as_str()),
+            ("invoke_type", "explicit"),
+        ],
     );
 }