I7b (audit-I25): nodeId exporter alias across --json catalogs

[espadonne]

Summary

Closes audit-I25 (the CLI side of I7b). Every `--json` exporter that emits the sequential `id` field now also emits `nodeId` projected from the server's opaque `node_id`. Scripts should prefer `nodeId` going forward; `id` stays for one release cycle before the v0.2.0 strip.

Paired with shithub#395 (I7b server — `author_id` stripped from issue/PR/comment/review responses).

What landed

• `internal/keys.SSHKey` — new `NodeID` field (the server-side I7a expansion landed `node_id` on the user-facing SSH-key surface).
• `pkg/cmd/sshkey/list` exporter — `nodeId` field added to the catalog + projection.
• `pkg/cmd/pr/list` exporter — same.
• `pkg/cmd/issue/list` exporter — same.
• `pkg/cmd/repo/view` exporter — same.

Behavior

• `shithub ssh-key list --json id,nodeId` returns both fields. `id` is the sequential integer; `nodeId` is the opaque base64-encoded `gid://shithub/SSHKey/{id}` from the server.
• Pre-deploy of I7a/I7b server (the field is server-emitted), `nodeId` is an empty string — graceful degradation rather than a hard fail.

Out of scope

• Sequential `id` field strip — v0.2.0 work. This bundle only adds the alias.
• README migration note — small follow-up doc PR.

Test plan

• Build + vet clean across all packages
• Targeted tests green for sshkey/list, pr/list, issue/list, repo/view
• No new lint regressions on my files
• Dogfood post-merge: `shithub ssh-key list --json id,nodeId` returns both fields populated

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