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/__tests__/LaneGitHeader.test.tsx

@@ -55,7 +55,7 @@ describe("LaneGitHeader", () => {
     expect(block.textContent).toContain("feat/rename-metric");
     expect(block.textContent).toContain("9b3e74a");
     expect(block.textContent).toContain("free-text rule mode in the form");
-    expect(block.textContent).toContain("2");
+    expect(screen.getByTestId("lane-git-counts").textContent).toContain("2");
   });
 
   it("drops the previous lane's facts the moment the lane changes", async () => {