Move skill catalog rendering out of core (#36364)

## What changed

- Make the skills extension own the catalog prompt templates and rendering path.
- Remove the duplicate core fallback that injected available skills into initial context.
- Update core integration tests to install the skills extension explicitly when they expect model-visible skill context.

GitOrigin-RevId: bbe6b15c4e776a09c98b7f4166426fe4e58434ba

codex-rs/ext/skills/src/render_tests.rs

@@ -8,7 +8,6 @@ use crate::provider::SkillProvider;
 use codex_core_skills::HostSkillsSnapshot;
 use codex_core_skills::loader::SkillRoot;
 use codex_core_skills::loader::load_skills_from_roots;
-use codex_core_skills::render_available_skills_body;
 use codex_exec_server::LOCAL_FS;
 use codex_extension_api::ContextualUserFragment;
 use codex_protocol::protocol::SkillScope;
@@ -19,6 +18,8 @@ use std::time::SystemTime;
 use std::time::UNIX_EPOCH;
 use tokio::sync::Semaphore;
 
+use crate::catalog_prompt::render_available_skills_body;
+
 fn entry(name: &str, description: &str, short_description: Option<&str>) -> SkillCatalogEntry {
     entry_with_path(
         name,