Add Bash, Zsh, and Fish completions

[phucnguyen1707]

Summary

• add moshcode completion bash|zsh|fish
• derive commands, engines, aliases, tools, MCP verbs, and skill verbs from shared registries
• preserve file fallback and respect nested command depth in each shell
• document profile setup for Bash, Zsh, and Fish

Tests

• npm test (491 tests, 0 failures)