feat(word-addin): run workflows and actions the way the web app does

The pane previously pasted workflow skill_md - and the Actions tab pasted
the whole document body - directly into the user message. The web app
never does either: a workflow travels as a `{ id, title }` reference the
backend resolves server-side (inside the semi-trusted
<workflow-instructions> fence), and document text reaches the model only
through nonce-fenced context blocks.

Pasting the body client-side quietly upgraded its trust level: a shared
workflow with a hostile body, or a document with embedded instructions,
arrived as the user's own words and bypassed the spotlighting the backend
builds for exactly this content. It also guaranteed behavioral drift -
server-side improvements to a workflow's prompt would never reach the
pane.

Now:
- WorkflowPicker sends the workflow reference on the message and lets the
  backend fetch and fence skill_md, same as the web.
- Proofread/Anonymise send the document via `document_context` (fenced
  server-side) with an instruction-only user message.
- The client uses the route's snake_case `document_context` field.

E2E request-body assertions updated to pin the new wire shape.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>