Add Codex CLI installation docs to README

[Copilot]

• P0: Add Codex CLI installation section to README.md (alongside existing Copilot CLI / Claude Code / VS Code sections)
• Add developers.openai.com to eng/reference-scanner/known-domains.txt so the reference scanner passes
• P3: Upstream feature request for Codex marketplace protocol support noted (external action)
• Run code review and security checks

Note: P1 (batch installation helper) and P2 (optional agents/openai.yaml) are intentionally deferred per maintainer guidance.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature Request: Add support for Codex</issue_title>
<issue_description>Currently dotnet/skills only supports Copilot CLI, Claude Code, and VS Code.
Please add official support for Codex.
This would make .NET skills available to Codex users.
Thanks!
</issue_description>

<agent_instructions>See the analysis in this issue. We shouldn't do P1 or P2 but P0 and P3.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ViktorHofer # Adding Codex Support to dotnet/skills

Analysis for dotnet/skills#386.

TL;DR

The skills in this repo already work with Codex at the format level — they use the same agentskills.io open standard (SKILL.md with YAML frontmatter). The gap is entirely in distribution and installation UX: Codex doesn't support the marketplace protocol that Copilot CLI, Claude Code, and VS Code use.

How Current Platforms Work

Copilot CLI, Claude Code, and VS Code all use a marketplace protocol:

1. The repo contains marketplace manifests (.github/plugin/marketplace.json, .claude-plugin/marketplace.json).
2. Users register the marketplace: /plugin marketplace add dotnet/skills
3. Users install a plugin (bundle of skills): /plugin install dotnet-msbuild@dotnet-agent-skills
4. All skills in that plugin are installed at once.

How Codex Works

Per the official Codex skills docs, Codex uses a fundamentally different model:

• No marketplace.json protocol. Codex doesn't read .github/plugin/marketplace.json or any equivalent.
• No "plugin" concept. Codex has individual skills, not plugin bundles.
• Directory-based discovery. Codex scans .agents/skills/ from CWD up to repo root, plus ~/.agents/skills/ (user-global) and /etc/codex/skills (admin).
• Per-skill installation via $skill-installer, which accepts a skill name (from openai/skills) or a GitHub URL.
• Optional agents/openai.yaml per skill for Codex-specific UI metadata (display name, icon, invocation policy, MCP tool dependencies).

What Already Works

• ✅ SKILL.md format is identical. Both Codex and Copilot CLI/Claude Code use the agentskills.io specification. Our skills are already compatible.
• ✅ Users can install individual skills today via $skill-installer with a GitHub URL, e.g.:

  $skill-installer install https://github.com/dotnet/skills/tree/main/plugins/dotnet-msbuild/skills/check-bin-obj-clash
  

• ✅ The repo already has .agents/skills/ with repo-level skills (create-skill, create-custom-agent).

What's Needed

P0: Documentation (small effort)

Add a Codex installation section to README.md alongside the existing Copilot CLI / Claude Code / VS Code sections. Example:

### Codex CLI

Skills in this repository follow the [agentskills.io](https://agentskills.io) open standard
and are compatible with [OpenAI Codex](https://developers.openai.com/codex/skills).

Install individual skills using `$skill-installer` with the GitHub URL:

    $skill-installer install https://github.com/dotnet/skills/tree/main/plugins/<plugin>/skills/<skill-name>

For example, to install the `check-bin-obj-clash` skill:

    $skill-installer install https://github.com/dotnet/skills/tree/main/plugins/dotnet-msbuild/skills/check-bin-obj-clash

P1: Batch installation helper (medium effort)

Since Codex can't install an entire "plugin" at once, consider providing a helper that installs all skills from a plugin into the user's .agents/skills/ directory. Options include:

• A shell script that symlinks or copies skills from a cloned repo
• A meta-skill (e.g., $dotnet-skills-installer) that automates the process
• Instructions for cloning the repo and symlinking the desired plugin's skills/ directory

P2: Optional agents/openai.yaml per skill (small effort per skill)

Add Codex-specific UI metadata for richer integration in the Codex app:

# plugins/dotnet-msbuild/skills/check-bin-obj-clash/agents/openai.yaml
interface:
  display_name: "Check Bin/Obj Clash"
  short_description: "Detects MSBuild projects with conflicting output paths"

policy:
  allow_implicit_invocation: true

This is optional — skills work without it — but improves discoverability in the Codex UI.

P3: Upstream request (external)

File a feature request with the Codex team to support a marketplace protocol (or similar bulk-install mechanism) so that repositories like dotnet/skills can offer a one-command installation experience comparable to Copilot CLI / Claude Code.

Key References

Resource	URL
Codex Skills docs	https://developers.openai.com/codex/skills
agentskills....

• Fixes Feature Request: Add support for Codex #386

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[ViktorHofer]

@copilot remove the example

[Copilot]

@copilot remove the example

Removed in 8b84503.

[Copilot]

Pull request overview

Adds documentation for installing skills from this repo using the Codex CLI, and updates the reference scanner allowlist so the new Codex documentation link passes validation.

Changes:

• Document Codex CLI installation via per-skill GitHub URL in README.md.
• Add developers.openai.com to the reference-scanner known domain allowlist.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Adds a “Codex CLI” installation section with a skill-installer example and link to Codex skills docs.
eng/reference-scanner/known-domains.txt	Allowlists developers.openai.com so the new README link passes the reference scanner.

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[JanKrivanek]

LGTM