fix(command-loader): strip incompatible fields before registering with OpenCode

Slash commands with arguments were silently failing in OpenCode TUI because
command definitions included 'name' and 'argumentHint' fields that don't exist
in OpenCode's Command schema. Strip these fields before registration across
all command/skill loaders to ensure compatibility.

Affected loaders:
- builtin commands
- claude-code command loader
- opencode skill loader
- claude-code plugin loader

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/claude-code-command-loader/loader.ts

@@ -62,7 +62,8 @@ $ARGUMENTS
 function commandsToRecord(commands: LoadedCommand[]): Record<string, CommandDefinition> {
   const result: Record<string, CommandDefinition> = {}
   for (const cmd of commands) {
-    result[cmd.name] = cmd.definition
+    const { name: _name, argumentHint: _argumentHint, ...openCodeCompatible } = cmd.definition
+    result[cmd.name] = openCodeCompatible as CommandDefinition
   }
   return result
 }