fix: cap gstack skill descriptions for codex

[cweill]

Motivation

Codex was skipping the root gstack skill because the frontmatter description in SKILL.md exceeded the local loader limit of 1024 characters. That made /Users/cweill/github/gstack/SKILL.md invalid even though the underlying skill content was otherwise fine.

What This PR Does

• Shortens the root gstack skill description in SKILL.md.tmpl while keeping the browse summary, adjacent-skill routing hints, and proactive-mode toggle guidance.
• Regenerates the local SKILL.md and Codex-side .agents/skills/gstack/SKILL.md outputs from the updated template.
• Adds a regression test that parses generated frontmatter and enforces the 1024-character description ceiling across generated skill files.

Testing

• bun test test/gen-skill-docs.test.ts test/skill-validation.test.ts
• Verified /Users/cweill/github/gstack/SKILL.md and .agents/skills/gstack/SKILL.md both parse to 932-character descriptions.
• codex exec -C /Users/cweill/github/gstack -s workspace-write "Reply with exactly OK."
  • Returned OK
  • Reported only the unrelated adapt YAML warning, and no longer flagged gstack as invalid

Review Focus

• Does the shortened root description still provide enough trigger coverage for browse usage and adjacent gstack skill suggestions?
• Is the new generator test the right place to guard the loader limit, or should that check live elsewhere in the skill validation suite?