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/opencode-skill-loader/loader.ts

@@ -126,7 +126,8 @@ function loadSkillsFromDir(skillsDir: string, scope: SkillScope): LoadedSkill[]
 function skillsToRecord(skills: LoadedSkill[]): Record<string, CommandDefinition> {
   const result: Record<string, CommandDefinition> = {}
   for (const skill of skills) {
-    result[skill.name] = skill.definition
+    const { name: _name, argumentHint: _argumentHint, ...openCodeCompatible } = skill.definition
+    result[skill.name] = openCodeCompatible as CommandDefinition
   }
   return result
 }