feat: add /cd command for changing working directory

[athal7]

Summary

• Add /cd <path> command to change the working directory mid-conversation
• Support !cd shell mode for directory navigation
• Add CdTool for agents to change directories programmatically
• Add POST /cd endpoint and plugin API (setDirectory) for programmatic use
• Fix TUI event routing to survive directory changes via GlobalBus
• Regenerate SDK with /cd endpoint and EventInstanceDirectoryChanged type

Fixes #2177