docs(agents): document the plugin agents user-scope gate

[F0Rextasy]

What

I reviewed the full diff; the documented discovery gate omitted the user-scope opt-in that silently drops plugin agents, so the docs now state the enabledProviders requirement and the omp-install exemption that the code already applies.

Why

Fixes #11362 (docs half; the code exemption already ships)

Testing

• Docs-only change; verified against packages/coding-agent/src/task/discovery.ts discoverAgents user-scope filter
• No code changed, no tests affected

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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