feat: expose ZCode skill/init/plugin commands and friendly-error TUI-only commands

[william0wang]

Summary

• Add skill and init to SLASH_COMMANDS (resolved by ZCode backend)
• Add loadPluginCommands() for dynamic plugin command discovery
• Friendly error for TUI-only commands (/mcp, /plugins, etc.)
• 17 new tests

[gemini-code-assist]

Code Review

This pull request implements plugin command discovery and slash-command handling for the ZCode ACP server, allowing dynamic plugin commands to be loaded at startup and integrated with static slash commands. It also updates slash-command interception to handle unsupported TUI commands with friendly errors and pass through backend-resolved commands. The review feedback highlights a critical bug in version resolution where version directories are compared lexicographically (e.g., treating "2.0.0" as greater than "10.0.0") and suggests implementing a semver-aware comparison helper. Additionally, the reviewer recommends using homedir() from node:os for robust, cross-platform home directory resolution in both the source code and tests.

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