[FEATURE]:Add opencode-vibeguard to ecosystem plugins list

[inkdust2021]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I'd like to add a community plugin to the Ecosystem → Plugins list.

Plugin: opencode-vibeguard

• GitHub: https://github.com/inkdust2021/opencode-vibeguard
• npm: https://www.npmjs.com/package/opencode-vibeguard

What it does:

• Redacts configured secrets/PII into VibeGuard-style placeholders before prompts/tool history are sent to LLM
  providers
• Restores placeholders locally on text-end
• Restores placeholders in tool args before execution

This is implemented purely as an external plugin (no core changes). If you're OK with it, I can open a PR adding one row to packages/web/src/content/docs/ecosystem.mdx.

Relates to #4961.