added codex plugin manifests and skill metadata

[blondak]

The Nette plugins now provide Codex-specific discovery and presentation metadata while sharing their existing skill instructions and references with Claude Code. Codex can discover all four plugins through the existing nette marketplace.

Changes:

• Added .codex-plugin/plugin.json manifests for nette, nette-dev, nette-lint, and php-fixer, including presentation metadata and starter prompts. Skill-bearing plugins explicitly reference ./skills/.
• Added agents/openai.yaml display names and short descriptions for all 24 skills.
• Preserved explicit-only invocation of install-php-fixer through Codex's allow_implicit_invocation policy and the existing Claude Code flag.
• Adapted PHP fixer guidance to distinguish verified Claude Code hook behavior from unverified Codex automation and document manual ECS use.
• Documented Codex marketplace registration, listing, installation, and skill invocation. Clarified that the catalog name is nette, independent of the checkout directory or branch, so the install ID is nette@nette.

Validation:

• Codex CLI listed all four plugins with --marketplace nette --available.
• Plugin validation passed for nette, nette-dev, and nette-lint.
• YAML UI metadata checks passed for all 24 skills.
• git diff --check passed.

Known limitations:

• Automatic linting and fixing hooks still target Claude Code Edit/Write events. Codex hook execution and end-to-end installation were not tested.
• The bundled Codex plugin validator rejects the preserved disable-model-invocation: true field in install-php-fixer; the php-fixer plugin needs an installation check in the target Codex client.
• The standalone skill validator also rejects that Claude-specific field and angle brackets in five existing skill descriptions. These existing instructions were preserved.