fix(run,lanes): prompt autocomplete, per-lane Active runs, compact Changes header

- The run form's prompt box offers suggestions again: `/` lists the run
  cwd's project and user commands and skills plus the CLI built-ins
  (claude lanes only), `@` lists files under the cwd; Enter picks while
  the list is open and sends otherwise. The 2f39f4e rewrite had left a
  bare textarea that ignored both.
- Active runs lists only the current lane's runs and external sessions
  (sessions matched to the lane by longest path-boundary cwd), instead of
  every lane's.
- The Changes panel's git line and its actions are one compact row in one
  card, and the mode/range toolbar is one wrapping row.
- The lane card hides itself when it has nothing to show, instead of
  leaving an empty frame where the git row used to be.

client/src/components/lanes/LaneCard.tsx

@@ -480,7 +480,9 @@ export default function LaneCard({
     <>
       <div
         data-testid={`lane-card-${lane.id}`}
-        className="flex flex-col rounded-lg border border-border bg-surface-4 p-2.5 transition-colors hover:border-border-light"
+        // Every row below is conditional: a lane with nothing to report
+        // renders no frame at all rather than an empty box.
+        className="flex flex-col rounded-lg border border-border bg-surface-4 p-2.5 transition-colors hover:border-border-light empty:hidden"
       >
         {(lane.ci_status || lane.tool === "codex") && (
           <div className="mb-2 flex flex-wrap items-center gap-1.5 text-[11px]">