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.
Introduce the first mutations, along with the shared machinery the later
issue and PR mutation slices reuse.
- `issue create` with --title/--body/--body-file/--assignee/--label/
--milestone/--fields, emitting `issue: { number, title, state, url }`
- `issue comment <n>`, echoing the created comment from the POST response
with the body cleaned and truncated at 800 chars
- body-source resolution (--body vs --body-file), label and milestone
name->ID lookup, repeatable flags, and the `joined`/`selectExtraFields`
field extractors
Label lookup pages until exhausted, since a repo with more labels than one
page would otherwise fail to resolve a valid name. The end-to-end tier seeds
a mixed-case label and milestone and passes both in a different case, so the
case-insensitive lookup is verified against live Gitea rather than only
against fixtures.
