Add Codex plugin support

[pdebruin]

Adds .codex-plugin/plugin.json and .agents/plugins/marketplace.json, enabling OpenAI Codex users to install with:

codex plugin marketplace add microsoft/Build-CLI

Includes full interface metadata (displayName, description, default prompts, brand color, privacy/TOS links) for plugin directory display. Follows the official Codex plugin spec and the MicrosoftDocs/mcp pattern.

cc @mikekinsman @TianqiZhang

[mikekinsman]

Tested the Codex plugin install flow on the desktop app — skill didn't auto-load via the plugin/marketplace path.

I tried installing this through the Codex desktop app's "Add marketplace" dialog (Plugins → Create → Add marketplace) pointing at the repo with add-codex-plugin as the git ref and .agents/plugins as the sparse path. The marketplace install either failed silently or didn't persist — the plugin didn't appear under Manage → Marketplace, Plugins, or Skills.

When I opened a new Codex chat with the repo as the working directory, Codex only picked up the .mcp.json (Learn MCP server) but didn't load the skill from skills/microsoft-build/SKILL.md. It fell back to manually exploring the repo files and trying to build the CLI from source rather than having the skill pre-loaded.

What worked: Adding the skill to .agents/skills/microsoft-build/SKILL.md in the repo root. Codex auto-scanned $REPO_ROOT/.agents/skills/ on session start, and the skill loaded immediately without needing the marketplace install. With that path, queries like "what build sessions are for Azure Cosmos DB?" went straight to the Build 2026 catalog.

Suggestion: Consider adding .agents/skills/microsoft-build/SKILL.md (can be a copy or symlink of skills/microsoft-build/SKILL.md) so the skill works for anyone who clones the repo and opens it in Codex, independent of the marketplace flow.

Scenario	Result
PR as-is (plugin manifest + skills/)	❌ Skill didn't auto-load
Added .agents/skills/microsoft-build/SKILL.md	✅ Skill auto-loaded, worked immediately

[TianqiZhang]

In Learn MCP repo, we used to use ./ to point the plugin folder to repo root, so we can re-use the mcp.json and skills folder that we already have at repo root.

I tested with latest codex CLI, this trick no longer works. Codex CLI requires us to put related files under a sub folder.