feat: add SkillPolicy to skill metadata and support allow_implicit_invocation (#11244)

Tested by setting the policy in agents/openai.yaml to true, false, and
leaving it unset (default).
```
policy:
  allow_implicit_invocation: false
```
<img width="847" height="289" alt="Screenshot 2026-02-09 at 3 42 41 PM"
src="https://github.com/user-attachments/assets/d3476264-3355-47cf-894a-4ffba53e3481"
/>

codex-rs/tui/src/chatwidget/tests.rs

@@ -434,6 +434,7 @@ async fn submission_prefers_selected_duplicate_skill_path() {
             short_description: None,
             interface: None,
             dependencies: None,
+            policy: None,
             path: repo_skill_path,
             scope: SkillScope::Repo,
         },
@@ -443,6 +444,7 @@ async fn submission_prefers_selected_duplicate_skill_path() {
             short_description: None,
             interface: None,
             dependencies: None,
+            policy: None,
             path: user_skill_path.clone(),
             scope: SkillScope::User,
         },