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/pty-run.js

@@ -388,7 +388,7 @@ function spawnRun(args) {
   else if (resumeSessionId || initialPrompt) {
     throw makeErr(
       "ERUNLIVE",
-      `lane ${laneId}'s pane is already running \`${tmux.paneCommand(id) || "a process"}\` — stop it before resuming a session or sending a prompt`
+      `workspace ${laneId}'s pane is already running \`${tmux.paneCommand(id) || "a process"}\` — stop it before resuming a session or sending a prompt`
     );
   }