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.

docs/LANES.md

@@ -540,8 +540,8 @@ sync" about a comparison that was never possible. The panel renders them as
 
 ### Acting on a lane's working copy: the git action row
 
-Under the git line, the Changes panel carries the per-repo git **writes** the
-dashboard offers (`POST /api/lanes/:id/git/:action`, see `docs/API.md`):
+On the same row as the git line — as icons, each naming itself in its tooltip —
+the Changes panel carries the per-repo git **writes** the dashboard offers (`POST /api/lanes/:id/git/:action`, see `docs/API.md`):
 
 | Button | Runs | Refuses when |
 | --- | --- | --- |