feat(plugin-sdk): a module cannot declare or be scoped to its own LLM provider/backend #16753

Gap

AutoBot's plugin and module system (autobot_shared/plugin_sdk/) has no link to LLM provider selection. A module cannot declare, or be scoped to, its own model backend. Every plugin uses whatever provider the platform or fleet has selected.

This is not a duplicate of:

• LLM run-mode: Full Local / Hybrid / Full Provider selectable in SLM #15494, the fleet-level mode selector, which lists per-module selection as an explicit non-goal;
• feat(a2a): AutoBot work exchange: instances take paid, anonymised jobs from other AutoBots through an opt-in plugin #16697, peer job-exchange monetization, which is a different mechanism.

It was found during the docs and positioning work in #16733 / #16734. Plugin business content lives in the separate plugin repositories; this issue covers only the platform capability.

Decision needed

Should a plugin be able to declare a model backend? Options: a manifest field naming an allowed provider or tier, resolved through the existing provider registry, with the platform policy able to override or deny it. Or keep provider choice platform-only.

Acceptance criteria (if approved)

• A plugin manifest can declare a preferred or required provider or tier. The loader resolves it through the provider registry, never with a hardcoded client.
• Platform policy can deny or override a plugin's declaration, and the effective backend is visible to the operator.
• A plugin that declares nothing keeps today's behaviour. Tested both ways.