[FEATURE] Support Claude Code slash command format for compatibility

What feature would you like to see?

Enable direct compatibility with Claude Code slash commands format.

Currently, Codex CLI uses a different (stricter) format for YAML frontmatter in slash commands (in .codex/prompts) compared to Claude Code's .claude/commands. This creates friction for users who want to switch between or use both tools.

Specific differences:

• argument-hint and other YAML frontmatter fields have stricter format requirements in Codex
• Claude Code interprets YAML frontmatter using LLM capabilities rather than strict parsing

Proposed solution:
Use LLM-based interpretation for YAML frontmatter (similar to how Claude Code handles it) rather than strict script-based parsing. This would allow the same command files to work in both tools.

Benefits:

1. Lower learning curve for Claude Code users migrating to Codex
2. Ability to maintain a single collection of slash commands that works in both tools
3. More forgiving/flexible command syntax

Additional information

Environment:

• Codex version: codex-cli 0.86.0
• Platform: Darwin 25.2.0 arm64 arm

Reference: Claude Code stores commands in .claude/commands/ with YAML frontmatter that includes fields like description, allowed-tools, and argument definitions. These are interpreted flexibly by the LLM rather than strictly parsed.