Preserve skill catalog entries under metadata pressure (#34732)

## Why

Long skill descriptions can consume the catalog's metadata budget before later
skills are listed, hiding otherwise usable skills from the model.

## What changed

- Reserve space for every skill's name and locator when those minimum lines fit.
- Distribute the remaining token or character budget across descriptions in
  round-robin order.
- Fall back to omitting entries only when the minimum catalog cannot fit.

## Testing

Added extension and production-turn coverage that verifies moderate budget
pressure keeps every catalog entry, shortens descriptions evenly, and avoids an
omission marker.

GitOrigin-RevId: 41971e4b47a86863b3839707b5cffcd8d83b888c

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

@@ -149,13 +149,15 @@ fn character_fallback_counts_multibyte_metadata_by_characters() {
 }
 
 #[test]
-fn catalog_emits_omission_marker_when_every_skill_exceeds_budget() {
+fn catalog_emits_omission_marker_when_every_minimum_skill_line_exceeds_budget() {
+    let oversized = entry(
+        "oversized",
+        &"x".repeat(MAX_CATALOG_SKILL_DESCRIPTION_CHARS),
+        /*short_description*/ None,
+    )
+    .with_display_path(format!("skill://{}", "x".repeat(512)));
     let catalog = SkillCatalog {
-        entries: vec![entry(
-            "oversized",
-            &"x".repeat(MAX_CATALOG_SKILL_DESCRIPTION_CHARS),
-            /*short_description*/ None,
-        )],
+        entries: vec![oversized],
         warnings: Vec::new(),
     };