refactor(opencode): restructure config and replace legacy plugins

Rewrites opencode.json, drops the with-context, websearch and terminal-bell
plugins, adds plugins that strip Claude-only commands and OMC MCP servers,
and vendors the impeccable, web-design-guidelines and frontend-workflow skills.

.config/opencode/plugin/strip-omc-mcp.ts

@@ -0,0 +1,15 @@
+import type { Plugin } from "@opencode-ai/plugin"
+
+const OMC_MCP_PREFIX = "oh-my-claudecode:"
+
+export const StripOmcMcpPlugin: Plugin = async () => ({
+  config: async (config) => {
+    if (!config.mcp) return
+
+    for (const name of Object.keys(config.mcp)) {
+      if (name.startsWith(OMC_MCP_PREFIX)) {
+        delete config.mcp[name]
+      }
+    }
+  },
+})