Stop re-exporting skill APIs from codex-core (#37984)

## What changed

- Import skill metadata and mention helpers directly from `codex-skills`.
- Import host skill loading types directly from `codex-skills-extension`.
- Make the remaining `codex-core` skills module private and remove its skill API re-exports, including the `HostSkillsService` re-export from `codex-core-api`.

GitOrigin-RevId: 26bf8fffd980df9c4d0c308d7e983ceab9a5df82

codex-rs/core/src/lib.rs

@@ -81,7 +81,6 @@ pub(crate) mod prompt_debug;
 pub use prompt_debug::build_prompt_input;
 pub(crate) mod mentions {
     pub(crate) use crate::plugins::build_connector_slug_counts;
-    pub(crate) use crate::plugins::build_skill_name_counts;
     pub(crate) use crate::plugins::collect_explicit_app_ids;
     pub(crate) use crate::plugins::collect_explicit_plugin_mentions;
     pub(crate) use crate::plugins::collect_tool_mentions_from_messages;
@@ -90,11 +89,7 @@ mod sandbox_tags;
 pub mod sandboxing;
 mod session_prefix;
 mod session_startup_prewarm;
-pub mod skills;
-pub(crate) use skills::HostSkillsService;
-pub(crate) use skills::SkillMetadata;
-pub(crate) use skills::build_skill_name_counts;
-pub(crate) use skills::collect_explicit_skill_mentions;
+mod skills;
 pub(crate) use skills::maybe_emit_implicit_skill_invocation;
 pub(crate) use skills::skills_load_input_from_config;
 mod stream_events_utils;