Add a skill invocation extension contributor (#32200)

## What changed

- Add `SkillInvocationContributor` and registration support to the extension API.
- Provide invocation callbacks with session-, thread-, and turn-scoped extension data, the turn ID, skill resource, and explicit or implicit invocation kind.
- Notify registered contributors when Codex observes a deduplicated implicit skill invocation.

## Testing

- Extend the extension registry round-trip test to cover skill invocation contributors.

GitOrigin-RevId: f428336728889d9bf4fba41a7dd8d3a6f9b9728c

codex-rs/ext/extension-api/src/contributors.rs

@@ -14,6 +14,7 @@ use crate::ExtensionData;
 mod context;
 mod mcp;
 mod prompt;
+mod skill_invocation;
 mod thread_lifecycle;
 mod tool_lifecycle;
 mod turn_input;
@@ -25,6 +26,8 @@ pub use mcp::McpServerContribution;
 pub use mcp::McpServerContributionContext;
 pub use prompt::PromptFragment;
 pub use prompt::PromptSlot;
+pub use skill_invocation::SkillInvocationInput;
+pub use skill_invocation::SkillInvocationKind;
 pub use thread_lifecycle::ThreadIdleInput;
 pub use thread_lifecycle::ThreadResumeInput;
 pub use thread_lifecycle::ThreadStartInput;
@@ -247,6 +250,23 @@ pub trait TokenUsageContributor: Send + Sync {
     }
 }
 
+/// Contributor for skill invocations observed by the host or an owning extension.
+///
+/// Implementations should treat the skill resource as an opaque identity and keep this callback
+/// cheap because it runs inline with skill loading or command dispatch.
+pub trait SkillInvocationContributor: Send + Sync {
+    /// Called after one explicit skill load or deduplicated implicit skill invocation is observed.
+    fn on_skill_invocation<'a>(
+        &'a self,
+        _input: SkillInvocationInput<'a>,
+    ) -> ExtensionFuture<'a, ()> {
+        Box::pin(async move {
+            let _self = self;
+            let _input = _input;
+        })
+    }
+}
+
 /// Extension contribution that exposes native tools owned by a feature.
 pub trait ToolContributor: Send + Sync {
     /// Returns the native tools visible for the supplied extension stores.