cli: engine-only surface — platform verbs move to the vos plugin

[hblee12294]

The `vos` CLI becomes purely the engine's CLI: `render` / `still` / `info` / `check` / `preview` / `versions` — local, deterministic, no account, no platform endpoints, no credential code.

The vos.so platform verbs (`fetch` / `push` / `pull`) move to the plugin package (`@vosso/vos-plugin` on npm), which lives next to the service they talk to. Nothing changes for users who install both packages: every non-engine verb is forwarded through a new delegate-on-unknown seam (same `run(argv)` contract as before, with the earlier package names kept as fallbacks), so `vos push` / `vos record` / etc. keep working unchanged — and new plugin verbs surface through `vos` without needing a release of this package.

Also in this change:

• `vos help` merges the plugin's exported `manifest` (verb list + summaries) when installed, or prints a one-line install hint when not
• `vos versions` gains a row for the installed plugin
• a lightweight host-range check: if the plugin's manifest declares a newer required host, a plain-words upgrade note is printed
• `vos voila ` remains a hidden alias

Smoke-tested as real npm installs (packed tarballs in a scratch project): merged help, doctor row, delegation, and the missing-plugin hint, plus `vos fetch` → `vos check` end-to-end through the plugin.

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