feat(agent): add document-targeted site tools

[DavidBabinec]

What changed

• Replaced the active-page-only site agent read surface with list_documents, read_document, and open_document covering pages, templates, and visual components.
• Moved the annotated HTML + compact CSS read renderer into @core/ai so browser-backed document reads and tests share one implementation.
• Updated prompts, capability/tool gates, heavy-result elision, docs, and tests; extracted document-routing helpers out of the executor.

Why

Agents could discover that shared chrome lived in an everywhere template, but had no valid tool to read that non-active document. That pushed them into invalid node/document id calls and retry loops.

Impact

Agents can now inspect any editable document by ref without switching the canvas, open a document only when visual context is needed, and use returned uid values for edits. read_page is removed with no compatibility shim.

Verification

• bun run build
• bun test
• bun run lint