feat: support the related issues API

[katayama8000]

Summary

Adds client support for the issue relation ("related issues") endpoints:

Method	Endpoint
getRelatedIssues(issueIdOrKey)	GET /api/v2/issues/:issueIdOrKey/relatedIssues
addRelatedIssue(issueIdOrKey, params)	POST /api/v2/issues/:issueIdOrKey/relatedIssues
removeRelatedIssue(issueIdOrKey, relatedIssueId)	DELETE /api/v2/issues/:issueIdOrKey/relatedIssues/:relatedIssueId

Types

• Entity.Issue.RelatedIssue extends Entity.Issue.Issue — the responses are plain Issue objects with an extra type field carrying the relation type, so extending Issue mirrors the wire format exactly.
• Types.IssueRelationType = "RELATES" — kept to what the API actually supports today. The server also defines a reserved relation type that is neither accepted nor returned yet; exposing it here would leak an unreleased feature into the client surface, so it is intentionally left out and can be added when it ships.
• Option.Issue.AddRelatedIssueParams = { targetIssueId: number }.

Usage

const related = await backlog.getRelatedIssues("SAMPLE-1");
//    ^? Entity.Issue.RelatedIssue[]   e.g. [{ id: 2, issueKey: "SAMPLE-2", type: "RELATES", ... }]

await backlog.addRelatedIssue("SAMPLE-1", { targetIssueId: 2 });
await backlog.removeRelatedIssue("SAMPLE-1", 2);

Verification

Unit tests were added for all three methods (the POST test also asserts the form-encoded request body).

The methods were additionally exercised against a real Backlog space, which confirmed:

• addRelatedIssue / removeRelatedIssue return the target issue plus type: "RELATES".
• The relation is undirected — after adding it from the source issue, getRelatedIssues on the target issue returns the source issue.
• Adding the same relation twice is idempotent (the list stays at one entry).
• Removing a relation that does not exist rejects with Error.BacklogApiError (HTTP 404), so no special handling is needed on the client side.

npm run typecheck, npm test, npm run lint and npm run fmt:check all pass.

Notes

The JSDoc comments reference the endpoint paths rather than developer.nulab.com URLs, because the documentation pages for these endpoints are not published yet. They should be swapped for the canonical URLs once the docs go live.

🤖 Generated with Claude Code

[mdkn]

LGTM!!!