Rule that the `body` extra field truncates at 500 chars like `issue view`,
resolving the spec's Principle 3 / Command Surface contradiction. Route the
`body` extractor through a new `truncatedBody()` FieldDef so `issue list`,
`issue create`, `pr list`, and `search` all present it exactly as the detail
views do, and add a `--full` flag to each to suppress truncation, keeping the
inline hint's "use --full" promise honest.
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.
