swarm-ui: widen the shell body for the issue report route

mara (#3877): issue report too narrow — the 8-column table was
clipped by .shell-body's 60em readable-line-length cap, which is right
for the rest of the UI's cards/forms but too tight for a wide table.

damocles diagnosed the root cause and scoped the shape (route
allowlist + an additive .shell-body-wide modifier, no prop plumbing
through App.tsx) before asking for a nod; mara then routed the actual
build to me. Implemented that shape as scoped: /issues opts into a
90em cap via WIDE_BODY_ROUTES, every other route keeps 60em untouched.

Verified with real headless-chromium screenshots at 1280px: the issue
report's full 8-column row (through 'transitively blocks') now fits
with no horizontal scroll, and a second screenshot of /agents confirms
every other route is unaffected.

frontend/packages/swarm-ui/src/shell/Shell.css

@@ -148,6 +148,15 @@
      having shipped something to walk back first. */
   animation: shell-page-enter 160ms ease;
 }
+/* Opt-in widening for a route that genuinely needs more than a
+   readable-line-length column (see Shell.tsx's `WIDE_BODY_ROUTES`).
+   Only overrides `max-width` — margin/padding/animation stay
+   `.shell-body`'s, so this reads as "the same body, just wider" rather
+   than a parallel layout to keep in sync. 90em still centers with
+   visible margins on a typical desktop viewport; it isn't full-bleed. */
+.shell-body-wide {
+  max-width: 90em;
+}
 @keyframes shell-page-enter {
   from {
     opacity: 0;