refactor: add resolveSequentialNumber and getCompleteIssueData to issues-api
Move resolveSequentialNumber out of the commands layer into issues-api.ts
where it belongs alongside the other issue data functions. Add IssueData
interface as the canonical JSON shape for a single issue, and
getCompleteIssueData which fetches issue record, sequential number, and
state in one call (with optional state override for mutation commands that
already know the new state). Add tests for both new exports.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>