test: Codex generation tests + CI + docs for multi-agent support

Tests (28 new):
- Codex output path routing, frontmatter validation (name+description only)
- No .claude/skills/ path leaks in Codex output (regression guard)
- /codex skill exclusion, hook→prose conversion, multiline YAML
- --host agents alias, dynamic template discovery
- Codex skill validation + $B command validation
- find-browse priority chain verification
- Replace static ALL_SKILLS list with dynamic filesystem scan

CI:
- Add Codex freshness check to skill-docs workflow

Docs:
- AGENTS.md: Codex-facing project instructions
- README: multi-agent installation section
- CONTRIBUTING: dual-host development workflow
- CHANGELOG: v0.9.0 multi-agent support entry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -56,6 +56,24 @@ Open Claude Code and paste this. Claude does the rest.
 
 Real files get committed to your repo (not a submodule), so `git clone` just works. Everything lives inside `.claude/`. Nothing touches your PATH or runs in the background.
 
+### Codex, Gemini CLI, or Cursor
+
+gstack works on any agent that supports the [SKILL.md standard](https://github.com/anthropics/claude-code). Skills live in `.agents/skills/` and are discovered automatically.
+
+```bash
+git clone https://github.com/garrytan/gstack.git ~/.codex/skills/gstack
+cd ~/.codex/skills/gstack && ./setup --host codex
+```
+
+Or let setup auto-detect which agents you have installed:
+
+```bash
+git clone https://github.com/garrytan/gstack.git ~/gstack
+cd ~/gstack && ./setup --host auto
+```
+
+This installs to `~/.claude/skills/gstack` and/or `~/.codex/skills/gstack` depending on what's available. All 21 skills work across all supported agents. Hook-based safety skills (careful, freeze, guard) use inline safety advisory prose on non-Claude hosts.
+
 ## See it work
 
 ```