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/bottom_pane/mod.rs

@@ -1332,6 +1332,7 @@ mod tests {
                 short_description: None,
                 interface: None,
                 dependencies: None,
+                policy: None,
                 path: PathBuf::from("test-skill"),
                 scope: SkillScope::User,
             }]),