Fetch a single Jira issue by key (e.g. "ACME-123"). Returns key, browse URL, summary, description, issue type, status, priority, assignee, reporter, labels, project, timestamps, acceptanceCriteria (plain text, one line per item; null when the instance has no Acceptance Criteria field or it is empty), and links (URLs from the description and the issue's web links, deduped; each has a kind — "figma" marks a design mockup). TIP: links with kind 'figma' are design mockups — open them with figma_frame_image / figma_frame_spec to check the build against the design. Uses the calling user's stored Jira credential — does not accept inline credentials. Returns an actionable error if Jira is not configured.