Add AI tool action subscriber and contextual system prompts

Problem/Motivation

AI Plus needs to translate server-side AI tool actions (entity persistence, workspace switching) into browser-side UI updates, and provide the AI agent with rich context about what the user is looking at (current entity, selected elements, view mode).

Previously, context injection was minimal (just entity type and ID) and UI reactions used the now-removed HTML injection pattern. The selected element context used basic identifiers without structural metadata, limiting the AI's ability to understand what it's operating on.