Remove the workbench-scoped Plugins settings section

[TheGreatAxios]

Summary

• Plugins are global-only for now (owner ruling: "let's just remove workbench level plugins for now and just do global, we'll add it back later"). A workbench's own Settings surface (General/Agents/Plugins/Notifications/...) no longer offers a Plugins section.
• The bench-level Plugins page (/plugins, reached from the sidebar) is untouched — it was already tenant/bench-scoped, not per-workbench, and remains the only place to connect a plugin/tool connector.
• Removed as dead code (only ever consumed by the deleted section): plugins-section.tsx, plugins-api.ts, mcp-servers-api.ts, and their tests. All three routes they called (/api/tenants/:tenantId/connections/..., /api/tenants/:tenantId/credentials/:id) are the same generic tenant-scoped routes the global Plugins page and Shared Settings' Connections/AI providers section already use — no backend change, nothing removed there.

What's left in place on purpose

• No stored data was dropped. Credential rows a workbench previously wrote through this section (scoped to that workbench's own tenant id) live in the same generic tenant-credentials table the rest of the app already reads/writes — nothing here reads or writes them anymore, but they are not deleted, so a future re-introduction of workbench-scoped plugins has something to resume from.
• Inference-provider connections (Anthropic, OpenAI, Ollama, ...) were never part of the workbench-scoped Plugins section to begin with — they live only in Shared Settings' "AI providers" section (packages/settings-ui), unaffected by this change.

Test plan

• bun run typecheck in packages/chat-ui
• bun test in packages/chat-ui (659 pass)
• Live-verified on the shared :3000 stack (current main) that the workbench Settings Plugins section exists today and that the global /plugins gallery and Settings → AI providers are unaffected by this diff (code-reviewed — this branch touches only packages/chat-ui/src/workbench-settings/*)

Linear: CL-6618