[codex] Preserve skill descriptions outside model context (#29006)

## Why

Skill descriptions are used in model-visible lists: the default
available-skills catalog that supports implicit selection, and the
on-demand `skills.list` tool response used to discover orchestrator
skills. A single overlong description should not consume a
disproportionate share of either list.

Enforcing the 1024-character limit while loading or migrating skills is
the wrong boundary: it rejects otherwise-valid skills and discards
metadata that non-model consumers and full skill reads may need. Skill
metadata and `SKILL.md` content should remain intact; the cap belongs at
model-visible list rendering boundaries.

## What changed

- Preserve full `description` and `metadata.short-description` values
when loading skills.
- Preserve full external-agent command descriptions during
`source-command-*` migration instead of skipping commands solely because
their descriptions exceed 1024 characters.
- Preserve full normalized orchestrator descriptions in the underlying
skills catalog.
- Cap each description at 1024 Unicode characters when rendering the
default available-skills context in `codex-core-skills` and
`codex-skills-extension`.
- Apply the same cap when serializing descriptions in the model-visible
`skills.list` response.
- Render truncated descriptions as 1021 original characters plus `...`.
- Leave explicit `$skill` injection, `skills.read`, underlying metadata,
and on-disk `SKILL.md` files unchanged and full-fidelity.

## Implicit skill selection

Codex injects a bounded catalog containing each implicitly allowed
skill's name, description, and source locator, together with
instructions to use a skill when the task clearly matches its
description. The model makes that semantic choice; after selecting a
skill, it reads the full `SKILL.md` from its filesystem or provider
resource. Explicit `$skill` mentions remain a separate path that injects
the full skill instructions. For orchestrator skills, `skills.list`
provides bounded discovery metadata before `skills.read` returns the
full selected resource.

## Test plan

- `just test -p codex-core-skills`
- `just test -p codex-skills-extension`
- `just test -p codex-external-agent-migration`

The focused regressions verify that overlong metadata is preserved at
load and migration boundaries while default available-skills rendering
and `skills.list` output produce the 1021-character prefix plus `...`.

codex-rs/external-agent-migration/src/lib.rs

@@ -18,7 +18,6 @@ const EXTERNAL_AGENT_HOOKS_SUBDIR: &str = "hooks";
 const EXTERNAL_AGENT_MIGRATED_HOOKS_SUBDIR: &str = "hooks";
 const COMMAND_SKILL_PREFIX: &str = "source-command";
 const MAX_SKILL_NAME_LEN: usize = 64;
-const MAX_SKILL_DESCRIPTION_LEN: usize = 1024;
 
 #[derive(Debug)]
 struct ParsedDocument {
@@ -1130,14 +1129,11 @@ fn command_skill_name_if_supported(
         return None;
     }
     let source_name = command_source_name(source_commands, source_file);
-    let description = command_skill_description(document, &source_name)?;
+    command_skill_description(document, &source_name)?;
     let name = command_skill_name(source_commands, source_file);
     if name.chars().count() > MAX_SKILL_NAME_LEN {
         return None;
     }
-    if description.chars().count() > MAX_SKILL_DESCRIPTION_LEN {
-        return None;
-    }
     if has_unsupported_command_template_features(&document.body) {
         return None;
     }
@@ -1388,6 +1384,8 @@ mod tests {
     use super::*;
     use pretty_assertions::assert_eq;
 
+    const MAX_SKILL_DESCRIPTION_LEN: usize = 1024;
+
     fn source_path(relative_path: &str) -> PathBuf {
         Path::new("/repo")
             .join(external_agent_config_dir())
@@ -1722,6 +1720,35 @@ command = "enabled-server"
         assert!(command_skill_name_if_supported(&root, &file, &document).is_none());
     }
 
+    #[test]
+    fn commands_with_overlong_descriptions_are_preserved() {
+        let root = source_path("commands");
+        let file = source_path("commands/review.md");
+        let description = "x".repeat(MAX_SKILL_DESCRIPTION_LEN + 1);
+        let document =
+            parse_document_content(&format!("---\ndescription: {description}\n---\nReview\n"));
+
+        assert_eq!(
+            command_skill_name_if_supported(&root, &file, &document),
+            Some("source-command-review".to_string())
+        );
+
+        let rendered = render_command_skill(
+            &document.body,
+            "source-command-review",
+            &description,
+            "review",
+        );
+        let rendered_document = parse_document_content(&rendered);
+        assert_eq!(
+            rendered_document
+                .frontmatter
+                .get("description")
+                .and_then(FrontmatterValue::as_scalar),
+            Some(description.as_str())
+        );
+    }
+
     #[test]
     fn commands_with_provider_runtime_expansion_are_skipped() {
         let root = source_path("commands");