Document the quick action rule on the save_work_item description

What does this MR do and why?

save_work_item rejects a description containing lines that start with / (Mcp::Tools::Concerns::ContentValidation), but its input schema said nothing about it, so an agent only found out from the runtime error "Quick actions (commands starting with /) are not allowed in description". save_note already documents the same rule on its body field.

This MR states the constraint in the description field's schema text and points at the structured fields (add_labels, assignee_ids, and so on) that cover what the quick actions would have done. The spec asserting the schema is updated to match.

References

• Resolves #628504 (closed)

Differences

Before: The description schema reads "Description in GitLab Flavored Markdown (max 1,048,576 characters)."

After: It continues: "Lines beginning with "/" are rejected to avoid triggering quick actions; use add_labels, assignee_ids and the other fields instead."

How to set up and validate locally

1. Call tools/list on the MCP endpoint and find save_work_item.
2. The inputSchema.properties.description.description text carries the new sentence.

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