feat(frontend): richer gate bodies + replay-friendly session exit

Improves how reviewer gates render their content, and adds two small
hooks that make the frontend cooperate with the standalone replay server
(also useful as generic primitives).

Gate body rendering:
- IntroBody: gate intros now render paragraphs plus real bullet lists
  with a hanging indent and emphasised bullet lead term, filling the
  card width (no narrow measure cap). Backed by introFormat (parser) +
  IntroBody (renderer), with tests.
- GateArtifactBody: artifact-review gates whose artifact is a bare file
  reference (e.g. {kind:"phase", data:{file:"question.md"}}) now fetch
  the referenced session document and render its full content, instead
  of showing the file reference as text. Falls back to ArtifactView for
  inline-content artifacts and unmapped files. Backed by gateArtifact
  (resolver) with tests. ArtifactGateWidget passes sessionId through.
- SelectWidget renders intros via IntroBody.

Replay-friendly session exit:
- AppShell: a system_event {event:"session_exit"} now triggers
  stopSession(), returning to the landing view. Generic primitive (the
  real backend can emit it too); used by the replay server's "Esci".
- SessionMenu: the ⋮ trigger is now always visible (was opacity-0 /
  group-hover only), so Resume is reachable without hovering.

Verified offline against the replay server (no VPN needed); tsc clean.

frontend/src/shell/WidgetHost.tsx

@@ -37,7 +37,7 @@ export function WidgetHost({ sessionId }: { sessionId: string | null }) {
   // widget render. resetKeys on the descriptor id so the next gate starts clean.
   return (
     <ErrorBoundary resetKeys={[pending.id]} label="step">
-      <Renderer descriptor={pending} onRespond={onRespond} />
+      <Renderer descriptor={pending} onRespond={onRespond} sessionId={sessionId ?? undefined} />
     </ErrorBoundary>
   );
 }