refactor: remove proxy admin endpoint (#13687)

## Summary
- delete the network proxy admin server and its runtime listener/task
plumbing
- remove the admin endpoint config, runtime, requirement, protocol,
schema, and debug-surface fields
- update proxy docs to reflect the remaining HTTP and SOCKS listeners
only

codex-rs/protocol/src/protocol.rs

@@ -2802,7 +2802,6 @@ pub struct SkillsListEntry {
 pub struct SessionNetworkProxyRuntime {
     pub http_addr: String,
     pub socks_addr: String,
-    pub admin_addr: String,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]