Linear Markdown shows node keys for issues it didn't fetch

Problem

issue-graph linear Markdown prints the full node key instead of the identifier for issues it knows about but doesn't fetch. This happens with the children of a hub that isn't expanded, issues past --max-nodes, and issues at the depth boundary.

## ENG-75 · Close the remaining parity gaps

Status: Backlog · hub not expanded

- → parent: linear:11111111-1111-4111-8111-111111111111:issue:00000000-0000-4000-8000-000000000101
- → parent: linear:11111111-1111-4111-8111-111111111111:issue:00000000-0000-4000-8000-000000000102
- → parent: ENG-94

The edge stores only the key, and renderLinear looks up labels from fetched nodes, so unfetched neighbors fall back to the key. Both Linear transports already return the related issue's identifier with the relation or child, so the reader has it.

Expected

• Unfetched neighbors render as their identifier, such as ENG-101, and are marked as not fetched.
• JSON and dashboard output keep the key as the identity.