Move host skill prompt injection into the skills extension (#37503)

## What changed

- Add `HostSkillsSnapshot::load_skill_prompts` so the skills extension owns reading and rendering selected host skills.
- Keep explicit invocation telemetry, unreadable-skill warnings, plugin prompt ordering, and suppression of host prompts superseded by provider prompts.
- Remove the legacy prompt injection and fragment implementation from `codex-core-skills`.

## Testing

- Cover prompt truncation without an installed skills extension, skill/plugin ordering, selective provider supersession, unreadable skills, and contextual fragment detection.

GitOrigin-RevId: 9c8b84ad54b90d174abc287ab49cc5a231e9e1aa

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

@@ -20,6 +20,16 @@ use crate::catalog_prompt::render_available_skills_body;
 use crate::loader::HostSkillRoot;
 use crate::loader::load_and_merge_host_skill_roots;
 
+#[test]
+fn skill_prompt_contents_are_bounded_at_utf8_boundaries() {
+    let contents = format!("{}é", "a".repeat(MAX_SKILL_PROMPT_BYTES - 1));
+
+    let (bounded, truncated) = truncate_main_prompt_contents(&contents);
+
+    assert_eq!(bounded.len(), MAX_SKILL_PROMPT_BYTES - 1);
+    assert_eq!(truncated, true);
+}
+
 fn entry(name: &str, description: &str, short_description: Option<&str>) -> SkillCatalogEntry {
     entry_with_path(
         name,