docs(skills): warn bare issue refs don't expand in epic notes

What does this MR do?

Adds a warning to the bundled glab skill: bare #123 issue references do not expand in notes on group-level items (epics, group work items).

Problem

Today an agent following this skill posted a comment on a group-level epic (&22419) containing bare issue references like #330331+. They rendered as literal text — no link, no title expansion — and the note needed a follow-up edit to fix.

The cause: issues are project-scoped, so GitLab resolves #123 against the current project context. Notes on group-level noteables have no project context, so the reference is left as plain text. Epic references (&123) are group-scoped and expand fine, which makes the failure easy to miss until it happens.

The skill's References line already teaches the cross-project form; this MR extends it to say the qualified group/project#123 form is required on group-level items, not just for cross-project links.

Related

The glab skill in the monolith (gitlab-org/gitlab, .claude/skills/glab/) is maintained separately and is getting the same fix in its own MR.