Add a Codex migration flow for toolkit skills and commands

[kimhoontae-gogo]

Summary

This PR adds an experimental Codex migration flow for claude-code-video-toolkit.

It keeps the existing Claude Code assets as the source of truth and adds a script-driven bridge for Codex users.

Included

• replace hardcoded local paths in command/docs examples
• add scripts/migrate_to_codex.py
• copy toolkit skills from .claude/skills/* into ~/.codex/skills
• generate command-wrapper skills from .claude/commands/*
• add a video-toolkit overview skill
• add --reset support to remove installed toolkit skills
• sync a generated Codex block from CLAUDE.md into repository AGENTS.md
• document the Codex flow in README.md, docs/getting-started.md, and CONTRIBUTING.md
• quote generated wrapper frontmatter values so installed skills remain valid YAML

Design

The migration flow keeps these files as the source of truth:

• .claude/commands/*
• .claude/skills/*
• CLAUDE.md

Codex-facing resources are generated from those sources rather than replacing them.

Validation

Tested locally with:

python3 -m py_compile scripts/migrate_to_codex.py
python3 scripts/migrate_to_codex.py --force
python3 scripts/migrate_to_codex.py --reset

Also verified:

- installed skills appear under ~/.codex/skills
- generated wrapper SKILL.md frontmatter parses as valid YAML
- a fresh Codex session recognizes the migrated video-toolkit skill
- AGENTS.md manual content is preserved outside the generated block

## Notes

- --reset removes toolkit-installed Codex skills and the generated Codex block in AGENTS.md
- --reset does not remove unrelated user skills from ~/.codex/skills
- manual AGENTS.md content outside the generated block is preserved