feat: add issue view with body cleaning and truncation (task 0003)

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.

src/cli.ts

@@ -13,6 +13,7 @@ const TOP_LEVEL_HELP = `usage: gitea-axi <command> [flags]
 
 commands:
   issue list    List issues in the current repository
+  issue view    Show a single issue's details
 
 global flags:
   -R, --repo <OWNER/NAME>   Override the repository detected from the git origin remote