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/lane-features.js

@@ -150,7 +150,7 @@ function archiveActiveFeature(laneId) {
 function activateFeature(laneId, rawSlug, options = {}) {
   const slug = canonicalizeSlug(rawSlug);
   const lane = lanesLib.getLane(laneId);
-  if (!lane) throw Object.assign(new Error(`no lane ${laneId}`), { code: "ENOLANE" });
+  if (!lane) throw Object.assign(new Error(`no workspace ${laneId}`), { code: "ENOLANE" });
 
   const current = lane.active_feature_id ? getFeatureById(lane.active_feature_id) : null;
   if (current && current.slug === slug) {