inline hostname resolution for remote sandbox config (#19739)

# Why

Requirements support host-specific
`remote_sandbox_config.hostname_patterns`, but config loading previously
resolved and passed the system hostname through every config-loading
path even when no requirements layer used `remote_sandbox_config`. On
machines where hostname lookup is slow, startup and app-server config
reads paid for a feature that was not active.

We only need the hostname when a requirements layer actually declares
`remote_sandbox_config`, so this moves hostname resolution to the single
requirements merge point and keeps all other config callers unaware of
hostname matching.

# What

- Removed the eager `host_name` plumbing from
`load_config_layers_state`, `load_requirements_toml`, `ConfigBuilder`,
app-server `ConfigManager`, network proxy loading, and related call
sites.
- Resolve the hostname inside
`merge_requirements_with_remote_sandbox_config` only when the incoming
requirements contain `remote_sandbox_config`.

codex-rs/core/src/session/handlers.rs

@@ -601,7 +601,6 @@ pub async fn list_skills(sess: &Session, sub_id: String, cwds: Vec<PathBuf>, for
             LoaderOverrides::default(),
             CloudRequirementsLoader::default(),
             &codex_config::NoopThreadConfigLoader,
-            /*host_name*/ None,
         )
         .await
         {