Update Workspace

Update a workspace's name, color palette index, and/or slug. Admin only. All fields are optional — supply only the fields you want to change. `slug` follows the same validation rules as on create (lowercase kebab-case, 1–50 characters, no reserved words). Returns 409 if the new slug is already claimed by another workspace, 422 if the slug format is invalid or reserved. Re-setting the workspace's current slug to itself never returns 409. Only workspace admins may call this endpoint; other members receive 404 (same as not-found, to avoid leaking membership details to non-members).