refactor(workspace): drop the page header, move Add lane and Tools onto the lanes

The title and the four counters said a second time what the lane strip
already shows lane by lane (liveness dot, ⚠ marker), so they only cost a row
of height on a screen whose point is the terminal. `GET /api/lanes` still
returns `counts` for other callers.

Add lane becomes the strip's FIRST tile — creating a lane is the same kind of
act as picking one — and the per-lane Tools menu (agents install / mcp sync /
sync-base check) moves onto the lane-detail header row beside the pipeline
picker and the Model select, next to the lane it acts on.

client/src/pages/__tests__/__snapshots__/screens.snapshot.test.tsx.snap

@@ -5097,42 +5097,20 @@ exports[`screen snapshots > Run 1`] = `
     class="space-y-5"
   >
     <div
-      class="flex flex-wrap items-center gap-3 border-b border-border pb-3"
+      class="flex min-h-0 flex-1 flex-col gap-4"
     >
-      <h2
-        class="text-base font-semibold tracking-tight text-fg-primary"
-      >
-        Lanes
-      </h2>
       <div
-        class="flex flex-wrap items-center gap-1.5 text-xs"
-      >
-        <span
-          class="rounded-full bg-surface-2 px-2.5 py-0.5 text-fg-secondary"
-          data-testid="count-total"
-        >
-          0
-           
-          lanes
-        </span>
-        <span
-          class="rounded-full bg-blue-600/20 px-2.5 py-0.5 text-blue-400"
-          data-testid="count-running"
-        >
-          0
-           
-          running
-        </span>
-      </div>
-      <div
-        class="ml-auto flex items-center gap-2"
+        class="flex shrink-0 flex-row gap-2 overflow-x-auto pb-1"
+        data-testid="lane-strip"
       >
         <button
-          class="flex items-center gap-1.5 rounded border border-border-light px-3 py-1 text-xs text-fg-secondary transition-colors hover:border-border-light hover:text-fg-primary"
+          class="flex w-56 shrink-0 items-center justify-center gap-1.5 rounded-lg border border-dashed border-border-light p-3 text-[13px] font-medium text-fg-secondary transition-colors hover:border-accent hover:bg-accent/5 hover:text-fg-primary"
+          data-testid="lane-tile-add"
           title="Create a lane from a working directory"
+          type="button"
         >
           <svg
-            class="lucide lucide-plus h-3.5 w-3.5"
+            class="lucide lucide-plus h-4 w-4"
             fill="none"
             height="24"
             stroke="currentColor"
@@ -5152,15 +5130,6 @@ exports[`screen snapshots > Run 1`] = `
           </svg>
           Add lane
         </button>
-      </div>
-    </div>
-    <div
-      class="flex min-h-0 flex-1 flex-col gap-4"
-    >
-      <div
-        class="flex shrink-0 flex-row gap-2 overflow-x-auto pb-1"
-        data-testid="lane-strip"
-      >
         <p
           class="text-sm text-fg-muted"
         >