Add standalone Codex skill installation

[pannal]

Fable is presented as a portable four-skill workflow, but the standalone installers currently target only Claude Code and copy only three skills. This PR adds a Codex install path, makes both installers copy the complete skill set, and tests both paths in isolated destinations.

Changes

• add explicit Claude Code and Codex targets to the Bash and PowerShell installers
• install all four skills, including fable-domain
• add agents/openai.yaml metadata and validate it in the repository checks
• fix the existing frontmatter incompatibilities so all four skills pass validation
• smoke-test both installer targets in isolated destinations

The frontmatter changes also resolve the angle-bracket validation failure reported in #8.

Closes #8

Verification

• python .github/checks.py
• skill validation for all four skills
• Bash installer tests for the default, Claude Code, Codex, and custom-destination paths
• Windows PowerShell installer tests for Claude Code and Codex with isolated destinations