save_work_item MCP tool should document quick action rejection in description field schema

Description

save_work_item's description field rejects content containing lines starting with / (quick actions), but the tool's schema doesn't document this constraint. save_note already documents it in its body field description:

Lines beginning with "/" are rejected to avoid triggering quick actions such as /merge

save_work_item should do the same for its description field so agents get the constraint from the schema rather than a runtime error.

Observed behavior

Input: {"project_id":"gitlab-org/caproni","work_item_iid":273,"description":"...content with /label ~foo...","add_labels":["foo"]}
Error: Validation error: Quick actions (commands starting with /) are not allowed in description

The error is correct — add_labels is the right way to set labels — but the schema gives no hint that / lines in the description are rejected.

Implementation plan

Add the same constraint note to the description field's schema description in save_work_item, e.g.:

Description in GitLab Flavored Markdown (max 1,048,576 characters). Lines beginning with "/" are rejected to avoid triggering quick actions.

Sibling issues for related MCP tool schema gaps: #618671 (closed), #618673 (closed), #621659.