feat(agent): add crab agent sync for cross-agent config migration

New command to sync user-level configurations between Claude Code and
Codex CLI:

- `crab agent status` — audit MCP servers and agents/skills on each side
- `crab agent sync mcp` — translate MCP server configs (JSON↔TOML)
- `crab agent sync agents` — LLM-assisted rewrite of agents↔skills
- `crab agent sync all` — sync everything

Supports --from claude|codex direction and --apply flag (dry-run by
default). MCP sync uses `codex mcp add` for reliable config writing.
Agent/skill rewrite uses whichever LLM CLI is available.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MrFlounder