fix: use codex co-author trailer in /ship

[malikrohail]

Summary

• make the /ship co-author trailer host-specific
• keep Claude output unchanged
• render an OpenAI Codex trailer in the generated Codex ship skill
• add generator coverage so the two hosts cannot drift back together

Problem

Issue #282 is still live on current main.

/ship hardcodes this trailer in the template:

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

That line is copied into the generated Codex ship skill today, so users running gstack under Codex get an Anthropic co-author in the final /ship commit message.

Fix

• add a host-aware COAUTHOR_TRAILER placeholder in ship/SKILL.md.tmpl
• render the Anthropic trailer for Claude output
• render Co-Authored-By: OpenAI Codex <noreply@openai.com> for Codex output
• add a regression test in test/gen-skill-docs.test.ts

Fixes #282.

Verification

• bun run gen:skill-docs
• bun run gen:skill-docs --host codex
• bun test test/gen-skill-docs.test.ts
• bun test

[malikrohail]

@garrytan verified on March 22, 2026 against current main.

I kept this intentionally small:

• one /ship template placeholder
• one generator resolver
• one generated Codex ship file update
• one regression test

No commit-message behavior outside /ship was changed in this PR. The goal here is just to stop Codex installs from emitting an Anthropic co-author trailer in the final /ship commit message.