feat: add Codex bootstrap support

[andrewjordancampbell]

Summary

• add a small Codex bootstrap path without touching the skill generator
• install Codex aliases under $HOME/.codex/skills/ via ./setup --host codex
• keep browse discovery Codex-aware so the installed binary resolves cleanly
• add minimal Codex-facing repo metadata and docs (AGENTS.md, agents/openai.yaml)

Why this PR is small

The previous Codex PR grew too large and too stale to review. This is the narrow first slice:

• setup --host codex
• Codex-aware find-browse
• minimal docs/metadata

I intentionally left generated Codex-native skill docs and template/generator work for a follow-up PR.

What changed

• setup
  • adds --host codex
  • creates gstack and gstack-* symlinks in $HOME/.codex/skills/
  • keeps the current Claude-layout helper paths available as compatibility symlinks so existing skill docs still run unchanged
• browse/src/find-browse.ts
  • prefers .codex/skills/gstack before .claude/skills/gstack
  • checks executability instead of mere existence
• browse/bin/find-browse
  • mirrors the same Codex-first discovery order in the shell fallback
• browse/test/find-browse.test.ts
  • adds Codex-first locator coverage for both workspace-local and global installs
• README.md / CONTRIBUTING.md
  • document the Codex install path briefly without broader docs churn
• AGENTS.md / agents/openai.yaml
  • add Codex-facing repo metadata

Testing

bun install
bash -n setup browse/bin/find-browse
bun test browse/test/find-browse.test.ts
bun test
TMP_HOME=$(mktemp -d)
HOME="$TMP_HOME" ./setup --host codex
HOME="$TMP_HOME" "$TMP_HOME/.codex/skills/gstack/browse/bin/find-browse"

Notes

• Existing Claude behavior is unchanged when ./setup is run without --host.
• This PR does not add generated .agents/skills/* output yet.
• The Codex bootstrap currently uses Claude-layout compatibility symlinks under ~/.claude/skills/ so the existing skill docs continue to work until the skill markdown itself becomes host-aware in a follow-up.

[andrewjordancampbell]

Closing this for now. I’m going to keep iterating on Codex support locally instead of asking for merge bandwidth upstream at the moment.