chore: remove skill metadata from command approval payloads (#15906)

## Why

This is effectively a follow-up to
[#15812](https://github.com/openai/codex/pull/15812). That change
removed the special skill-script exec path, but `skill_metadata` was
still being threaded through command-approval payloads even though the
approval flow no longer uses it to render prompts or resolve decisions.

Keeping it around added extra protocol, schema, and client surface area
without changing behavior.

Removing it keeps the command-approval contract smaller and avoids
carrying a dead field through app-server, TUI, and MCP boundaries.

## What changed

- removed `ExecApprovalRequestSkillMetadata` and the corresponding
`skillMetadata` field from core approval events and the v2 app-server
protocol
- removed the generated JSON and TypeScript schema output for that field
- updated app-server, MCP server, TUI, and TUI app-server approval
plumbing to stop forwarding the field
- cleaned up tests that previously constructed or asserted
`skillMetadata`

## Testing

- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-protocol`
- `cargo test -p codex-app-server-test-client`
- `cargo test -p codex-mcp-server`
- `just argument-comment-lint`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1705,7 +1705,6 @@ mod tests {
                 }),
                 macos: None,
             }),
-            skill_metadata: None,
             proposed_execpolicy_amendment: None,
             proposed_network_policy_amendments: None,
             available_decisions: None,
@@ -1716,31 +1715,4 @@ mod tests {
             Some("item/commandExecution/requestApproval.additionalPermissions")
         );
     }
-
-    #[test]
-    fn command_execution_request_approval_skill_metadata_is_marked_experimental() {
-        let params = v2::CommandExecutionRequestApprovalParams {
-            thread_id: "thr_123".to_string(),
-            turn_id: "turn_123".to_string(),
-            item_id: "call_123".to_string(),
-            approval_id: None,
-            reason: None,
-            network_approval_context: None,
-            command: Some("cat file".to_string()),
-            cwd: None,
-            command_actions: None,
-            additional_permissions: None,
-            skill_metadata: Some(v2::CommandExecutionRequestApprovalSkillMetadata {
-                path_to_skills_md: PathBuf::from("/tmp/SKILLS.md"),
-            }),
-            proposed_execpolicy_amendment: None,
-            proposed_network_policy_amendments: None,
-            available_decisions: None,
-        };
-        let reason = crate::experimental_api::ExperimentalApi::experimental_reason(&params);
-        assert_eq!(
-            reason,
-            Some("item/commandExecution/requestApproval.skillMetadata")
-        );
-    }
 }