Draft: Add 19.3 release note for /goal command in Duo CLI

James Caseyrequested to merge
docs/19-3-goal-command-duo-cli into master

What does this MR do?

Adds the GitLab 19.3 release note for the new /goal slash command in Duo CLI, split out of !250241 (merged) so it doesn't block the other 19.3 AI Clients release notes.

Related issues

Blocked on

Waiting on docs MR !249993, which still has an open question on ship timing (which LSP release, when it's available to users) and placeholder version numbers in its history entry. Do not merge before that's resolved.

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