feat(agent): add crab agent sync for cross-agent config migration (#55)

## Summary

- **`crab agent status`** — audit what MCP servers and agents/skills
exist on Claude vs Codex side
- **`crab agent sync mcp`** — mechanically translate MCP server configs
between Claude's JSON and Codex's TOML (uses `codex mcp add` for
reliable config writing)
- **`crab agent sync agents`** — LLM-assisted rewrite of Claude agents →
Codex skills (and vice versa)
- Dry-run by default, `--apply` to write changes, `--from claude|codex`
for direction

Builds on the agent-agnostic foundation from #52.

## Test plan

- [x] `crab agent status` — correctly shows Linear MCP in Claude, agents
list, gaps
- [x] `crab agent sync mcp --from claude --apply` — synced Linear to
Codex, verified with `codex mcp list`
- [x] `crab agent status` after sync — shows MCP "✓ In sync"
- [x] `make test-unit` — all 36 existing tests pass (no regressions)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: MrFlounder