feat(ui): call lanes "workspaces" in the UI, CLI output and docs

Copy-only rename (workspace redesign, phase 0). The word a user reads
changes — "Lane" → "Workspace", "Add lane" → "Add workspace" — in both
locales, in agenthub CLI prose and help, in the server's error messages
and in the docs. Every identifier stays: /api/lanes, `agenthub lanes`,
--lane, AGENTHUB_LANE_ID, error codes, tmux names, DB tables, file,
component and i18n key names. docs/LANES.md keeps its name, retitled
"Workspaces (lanes)".

client/src/components/run/LaneConsolePane.tsx

@@ -425,7 +425,7 @@ export function LaneConsolePane({
 
           const startedId = laneStartResult.run_id ?? laneStartResult.lane?.run_id;
           if (!startedId) {
-            throw new Error("No run_id returned from lane start");
+            throw new Error(t("errors.noRunIdReturned"));
           }
 
           fetched = await api.run.get(startedId);