Add `issue view <n>` as the first detail command, introducing the
content-cleaning and truncation machinery (src/body.ts) that later issue
and PR slices reuse.
- Default output: number, title, state, author, created, body (truncated
at 500), plus comment_count; --comments expands every comment (bodies
truncated at 800); --full suppresses all truncation.
- cleanBody runs only when a body exceeds its limit: normalizes Gitea
issue/PR URLs on the detected host to Issue#N/PR#N, strips image embeds
and long URLs, and collapses quoted blocks.
- Type guard: a PR number fails with VALIDATION_ERROR and a `pr view <n>`
hint, detected via the fetched object's pull_request field.
- renderDetail joins the detail entity, optional sub-blocks, and help.
