[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/core-skills/src/loader.rs

@@ -684,13 +684,8 @@ async fn parse_skill_file(
 
     validate_len(&base_name, MAX_NAME_LEN, "name")?;
     validate_len(&name, MAX_QUALIFIED_NAME_LEN, "qualified name")?;
-    validate_len(&description, MAX_DESCRIPTION_LEN, "description")?;
-    if let Some(short_description) = short_description.as_deref() {
-        validate_len(
-            short_description,
-            MAX_SHORT_DESCRIPTION_LEN,
-            "metadata.short-description",
-        )?;
+    if description.is_empty() {
+        return Err(SkillParseError::MissingField("description"));
     }
 
     let resolved_path = canonicalize_for_skill_identity(fs, path).await;