fix: keep root gstack skill loadable in codex

[malikrohail]

Summary

• keep the root gstack skill description under Codex CLI's current 1024-character frontmatter limit
• add a regression test covering the two root skill surfaces Codex loads today: SKILL.md and .agents/skills/gstack/SKILL.md
• preserve the existing root-skill behavior while trimming only the overlong frontmatter text

Why this PR

On March 22, 2026, I tested current main with codex-cli 0.116.0 and reproduced a live Codex compatibility error:

failed to load skill /Users/test/.codex/skills/gstack/SKILL.md: invalid description: exceeds maximum length of 1024 characters

I also saw the same root-skill failure for:

/Users/test/.codex/skills/gstack/.agents/skills/gstack/SKILL.md

I could not reproduce the symlink-specific claim from issue #333 on current main, but this root-skill frontmatter failure is a confirmed Codex loading bug on the current codepath.

PR #326 does not address this area. It does not touch the root gstack skill frontmatter or Codex root-skill loading.

Related to #333.

Verification

• bun test
• bun test test/gen-skill-docs.test.ts
• ./setup --host codex
• codex exec --ephemeral -C /Users/test/.codex/skills/gstack -s read-only -o /tmp/gstack-codex-skills-after.txt "List any installed skills you have available. Return only names, comma-separated."

Result

Before this change, Codex emitted the root gstack invalid description error.

After this change:

• gstack appears in the installed skill list
• the root gstack invalid description: exceeds maximum length of 1024 characters error is gone
• the new static regression test keeps both root skill descriptions under the current Codex limit

[malikrohail]

@garrytan tested on March 22, 2026 with codex-cli 0.116.0.

This is intentionally the smallest fix I could verify end-to-end:

• no setup redesign
• no new Codex metadata generation
• no broad skill rewrites
• just bring the root gstack skill description under Codex's current 1024-character frontmatter limit
• lock it with a regression test for both SKILL.md and .agents/skills/gstack/SKILL.md

Before this patch, Codex emitted the root-skill load error during a real ./setup --host codex + codex exec flow. After this patch, gstack loads successfully and that specific invalid description error is gone.

I also checked PR #326 while narrowing this down. It does not touch the root gstack frontmatter or Codex root-skill loading path, so it does not solve this bug.