Enable user-defined commands in TUI configuration

[Copilot]

This PR implements user-defined commands support in the OpenCode TUI, allowing users to extend the built-in command set through configuration. This addresses the requirement to enable user-defined commands similar to those in packages/tui/internal/commands/command.go.

What's Changed

Core Implementation

• Extended Config struct with commands field to support user-defined commands
• Modified LoadFromConfig() to load and merge user commands with built-in defaults
• Fixed parseBindings() function to properly handle leader key detection with whitespace
• Added ConfigCommand type with support for descriptions, keybindings, and triggers

Configuration Format

Users can now define custom commands in their opencode.json:

{
  "commands": {
    "my_command": {
      "description": "My custom command",
      "keybind": "<leader>x",
      "trigger": ["custom", "mycmd"]
    },
    "quick_note": {
      "description": "Take a quick note", 
      "keybind": "ctrl+shift+n",
      "trigger": ["note", "quick"]
    }
  }
}

Features Supported

• Custom descriptions for user commands
• Flexible keybindings including leader key combinations and multi-key bindings
• Text triggers for command activation (e.g., /custom, /note)
• Integration with command completions and fuzzy search
• Coexistence with built-in commands without conflicts

Integration Points

• User commands appear in command completions when typing /
• Keybindings work seamlessly with the existing input system
• Commands are included in help and command listings
• Fuzzy search finds user commands alongside built-in ones

Testing

Added comprehensive test coverage with 9 test cases:

• Basic user command loading and configuration
• Complex keybinding scenarios with leader keys
• Integration with command registry and sorting
• Completions provider integration
• Conflict prevention between user and built-in commands
• Empty configuration handling
• Custom keybinding application

Documentation

• Created USER_DEFINED_COMMANDS.md with detailed usage instructions
• Provided example configuration file demonstrating all features
• Documented keybinding format and configuration options

Backward Compatibility

✅ Fully backward compatible - existing configurations continue to work unchanged
✅ All existing commands and keybindings remain functional
✅ No breaking changes to the command system API

Example Usage

After adding commands to config, users can:

1. Type /custom to trigger the command via text
2. Press Ctrl+X x (leader + x) to trigger via keybinding
3. See commands in completions when typing / and searching
4. View in help alongside built-in commands

This provides a solid foundation for future extensibility where users will be able to define custom behavior for their commands.

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