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/pages/Workspace.tsx

@@ -874,7 +874,8 @@ export function Workspace() {
             )}
             {currentLane && showInfo && (
               <div className="max-h-[45vh] shrink-0 overflow-y-auto">
-                <div className="mb-3">
+                {/* No margin when the card rendered nothing (see LaneCard). */}
+                <div className="mb-3 has-[>div:empty]:mb-0">
                   <LaneCard
                     lane={currentLane}
                     onAction={(a, b) => handleLaneAction(currentLane.id, a, b)}