group activity entries by plan step + remove inline renderers from chat

- Tool calls in Activity timeline are now tagged with current plan step
- Indented entries show which step they belong to
- Step tracking via update_plan_step(status=running) tool calls
- Removed inline tool renderers from chat (health summary, fleet snapshot, etc.)
  — all tool output now visible only in sidebar Activity timeline

web/src/lib/components/ActivityTimeline.svelte

@@ -59,7 +59,7 @@
             {/if}
             <button
               type="button"
-              class="flex w-full items-start gap-2 px-3 py-1.5 text-left text-xs hover:bg-muted/30 {hasDetail ? 'cursor-pointer' : 'cursor-default'}"
+              class="flex w-full items-start gap-2 {entry.indent ? 'pl-7' : 'px-3'} py-1.5 text-left text-xs hover:bg-muted/30 {hasDetail ? 'cursor-pointer' : 'cursor-default'}"
               onclick={() => hasDetail && toggle(entry.id)}
             >
               <!-- status icon -->