fix(cosh): add auto-completion for /statusline subcommands

[samchu-zsl]

Description

Refactored the /statusline command to use the SlashCommand subcommand registration pattern. Previously, /statusline parsed arguments manually with string matching, which meant the auto-completion framework had no awareness of its show and clear subcommands. Now these are registered as proper subCommands, enabling the existing completion engine to suggest them automatically when users type /statusline.

Related Issue

closes #406

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• CI/CD or build changes

Scope

• cosh (copilot-shell)

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• For cosh: Lint passes, type check passes, and tests pass
• Lock files are up to date (package-lock.json / Cargo.lock)

Testing

cd src/copilot-shell
npm run build && npm run test && npm run lint && npm run typecheck
# All 178 test files passed (3858 tests), lint clean, typecheck clean

Additional Notes

• clear subcommand retains off as an altName for backward compatibility
• Parent action still handles arbitrary command strings (e.g., /statusline echo "hello") for setting the status line command
• No public API or configuration format changes

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.

[samchu-zsl]

Caution

This PR was generated and submitted by AI.
Please review all changes carefully before merging. Pay special attention to logic correctness, edge cases, and potential side effects.

[samchu-zsl]

已手动测试，符合显示子命令的要求

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.

[KaiLongZhou]

LGTM