refactor: split cli/commands.py into focused modules

- Extract uninstall functionality to uninstall_commands.py (299 lines)
  - UninstallContext dataclass, TOOL_CONFIG_DIRS, NPM_PACKAGE_MAP
  - Helper functions for backup, confirmation, and tool removal

- Extract shell completion to completion_commands.py (666 lines)
  - Bash and Zsh completion script generation
  - Dynamic command/option completion logic

- Reduce commands.py from 1338 to 465 lines
  - Core commands: upgrade, install, doctor, version, launch
  - Backward-compatible re-exports from new modules

- Fix test_cli.py: add missing keep_config parameter to uninstall tests

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

Author: zhujian0805