feat: truncate --fields body uniformly (task 0021)

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.

src/commands/label.ts

@@ -124,7 +124,9 @@ async function labelList(deps: CliDeps, args: string[]): Promise<string> {
   }
 
   const now = new Date();
-  const rows = labels.map((label) => extractRow(label, LABEL_LIST_FIELDS, { now }));
+  const rows = labels.map((label) =>
+    extractRow(label, LABEL_LIST_FIELDS, { now, host: context.host, full: false }),
+  );
   return renderList({
     noun: "labels",
     rows,