feat: add related issues tools

[katayama8000]

Summary

Adds tools for the related issues API to the issue toolset, backed by the client methods that shipped in backlog-js v0.19.0 (nulab/backlog-js#181).

Tool	Endpoint
get_related_issues	GET /api/v2/issues/:issueIdOrKey/relatedIssues
add_related_issue	POST /api/v2/issues/:issueIdOrKey/relatedIssues
remove_related_issue	DELETE /api/v2/issues/:issueIdOrKey/relatedIssues/:relatedIssueId

• Bumps backlog-js to ^0.19.0.
• Adds RelatedIssueSchema — an issue plus the type field the API returns alongside it.
• All three tools accept issueId / issueKey through the existing resolveIdOrKey helper, consistent with the other issue tools.

Scope

The API currently supports only a single, undirected relation type, so the tools intentionally expose no relation-type argument and RelatedIssueSchema.type is a plain string rather than an enum. This keeps the tool surface to what the API actually supports today, so nothing has to change here when the relation model is extended.

add_related_issue takes a numeric targetIssueId because that is what the endpoint accepts. Resolving a target issue key to an ID would need an extra getIssue call, so it is left out for now and can be added if it turns out agents need it.

Verification

• Unit tests for all three tools (12 cases): return value shape, issueKey / issueId pass-through, and the error when neither is given.
• Full suite: 419 tests pass. typecheck, typecheck:all, lint and format are clean.
• The tool handlers were also run against a real Backlog space, confirming the tools return the related issue with type: "RELATES" and that remove_related_issue / add_related_issue round-trip correctly.

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