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)".

server/lib/router-apply.js

@@ -193,7 +193,7 @@ function applyRouterToLane(lane, profileName) {
   // AI tool runs on its vendor's own settings, so native is the one fit.
   if (!ROUTER_CAPABLE.has(lane.tool || "claude")) {
     if (profile.builtin) return { tool: lane.tool, file: null, profile: profile.name };
-    throw Object.assign(new Error(`a ${lane.tool} lane cannot use an AI Router`), {
+    throw Object.assign(new Error(`a ${lane.tool} workspace cannot use an AI Router`), {
       code: "EUNSUPPORTED",
     });
   }