feat: remove redundant skill tool - OpenCode handles natively

OpenCode has native skill support that automatically scans .claude/skills/
and injects available_skills into system prompt. The agent reads SKILL.md
files directly via the Read tool, making our separate skill tool a duplicate.

The claude-code-skill-loader feature (which converts skills to slash
commands) is intentionally kept - only the redundant skill tool is removed.

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

Author: code-yeongyu

src/tools/index.ts

@@ -20,7 +20,6 @@ import {
 import { grep } from "./grep"
 import { glob } from "./glob"
 import { slashcommand } from "./slashcommand"
-import { skill } from "./skill"
 
 export { interactive_bash, startBackgroundCheck as startTmuxCheck } from "./interactive-bash"
 export { getTmuxPath } from "./interactive-bash/utils"
@@ -64,5 +63,4 @@ export const builtinTools = {
   grep,
   glob,
   slashcommand,
-  skill,
 }