Finish the `issue list` flag surface left minimal by the tracer slice:
- Server-side filters `--label`, `--assignee`, `--author`, `--milestone`,
mapped to Gitea's `labels`, `assigned_by`, `created_by`, `milestones`.
- Client-side `--sort <created|updated|comments>`, always descending, over
the fully paginated set (ADR 0005); `--limit` caps the sorted result.
- `--fields` exposing body, closedAt, labels, milestone, updatedAt, url.
- `--search` refused with a VALIDATION_ERROR redirecting to `search issues`.
Exhaustive pagination lands as a shared `paginate.ts`, since ADR 0005 makes
it a policy later slices reuse; `lookup.ts` drops its hand-rolled copy of the
same loop. The helper carries the 20-page cap that copy encoded, matching the
1000-item ceiling Principle 8 sets.
