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/proof.js

@@ -166,14 +166,14 @@ function proofFile(laneId, slug, group, file) {
  */
 function deleteProof(laneId, { slug, group, images } = {}) {
   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" });
   if (badName(slug)) throw Object.assign(new Error("invalid slug"), { code: "EBADPATH" });
 
   let base;
   try {
     base = fs.realpathSync(proofRoot(lane));
   } catch {
-    throw Object.assign(new Error("no proof for this lane"), { code: "ENOFEATURE" });
+    throw Object.assign(new Error("no proof for this workspace"), { code: "ENOFEATURE" });
   }
   const inBase = (p) => p === base || p.startsWith(base + path.sep);