docs: list opencli-plugin-multcloud in the community plugin lists

[vecyang1]

Adds one row to the community plugin lists in docs/guide/plugins.md and docs/zh/guide/plugins.md. Docs only — no code changes.

opencli-plugin-multcloud drives MultCloud through the OpenCLI browser bridge: cloud-to-cloud transfer, sync, backup, offline remote upload (HTTP/magnet), Cloud Explorer and task observability, across Google Drive, OneDrive, Baidu Netdisk, Google Photos, Gmail and the rest. MultCloud has no public API — payloads are AES-encrypted client-side and posted to /action.php with a session cookie — so it drives the real app in an authenticated tab.

opencli plugin install github:vecyang1/opencli-plugin-multcloud
opencli multcloud doctor

Twelve commands: account, stats, doctor, clouds, explorer, tasks, transfer, sync, backup, remote-upload, flow, shares.

A few things it does that might be useful precedent for other adapters against stateful web apps:

• Writes are verified by read-back. A create confirms a new task id appeared; a delete confirms the id is gone. The page reporting success is not treated as evidence.
• Unreadable values are null, never 0. Aggregates carry a warnings array on stderr, so -f json stays clean while a human still learns a total is a floor.
• 118 hermetic tests — no browser, no network, no account — behind a transport seam, plus an opt-in read-only live suite.

Licensing, since it matters for a docs listing: the plugin lives in its own repository under AGPL-3.0-or-later and depends on @jackwener/opencli as a peer dependency. Nothing AGPL is proposed for this repo — this PR is a one-line-per-file docs addition, contributed under Apache-2.0 per CONTRIBUTING.md, exactly like the other entries in those lists.

Happy to adjust the wording or drop the zh entry if you would rather keep that list shorter.