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/render.ts

@@ -28,6 +28,11 @@ export function formatCountLine(
   return `count: ${shown} of ${total} total`;
 }
 
+/** Encode a named list block, with an explicit empty-state line when there are no rows. */
+function encodeRows(noun: string, rows: Record<string, unknown>[]): string {
+  return rows.length > 0 ? encode({ [noun]: rows }) : `${noun}[0]: (none)`;
+}
+
 export interface RenderListOptions {
   noun: string;
   rows: Record<string, unknown>[];
@@ -36,9 +41,28 @@ export interface RenderListOptions {
 }
 
 export function renderList(options: RenderListOptions): string {
-  const body =
-    options.rows.length > 0
-      ? encode({ [options.noun]: options.rows })
-      : `${options.noun}[0]: (none)`;
+  const body = encodeRows(options.noun, options.rows);
   return [options.countLine, body, encode({ help: options.help })].join("\n");
 }
+
+/** A secondary list block appended below a detail entity (e.g. comments). */
+export interface DetailBlock {
+  noun: string;
+  rows: Record<string, unknown>[];
+}
+
+export interface RenderDetailOptions {
+  noun: string;
+  item: Record<string, unknown>;
+  blocks?: DetailBlock[];
+  help: string[];
+}
+
+export function renderDetail(options: RenderDetailOptions): string {
+  const parts = [encode({ [options.noun]: options.item })];
+  for (const block of options.blocks ?? []) {
+    parts.push(encodeRows(block.noun, block.rows));
+  }
+  parts.push(encode({ help: options.help }));
+  return parts.join("\n");
+}