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-runtime.js

@@ -59,7 +59,7 @@ function requireProfile(lane) {
   if (!profile) {
     throw Object.assign(
       new Error(
-        `lane has no .agenthub/profile — looked in: ${profileSearchPaths(lane).join(", ")}`
+        `workspace has no .agenthub/profile — looked in: ${profileSearchPaths(lane).join(", ")}`
       ),
       { code: "ENOPROFILE", searched: profileSearchPaths(lane) }
     );
@@ -176,7 +176,7 @@ function resolveLaneDirEntry(lane, entry) {
   const target = path.resolve(lane.cwd, entry);
   const relative = path.relative(lane.cwd, target);
   if (relative.startsWith("..") || path.isAbsolute(relative)) {
-    throw Object.assign(new Error(`LANE_DIRS entry escapes the lane: ${entry}`), {
+    throw Object.assign(new Error(`LANE_DIRS entry escapes the workspace: ${entry}`), {
       code: "EBADLANEDIR",
       entry,
     });