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/git-ops.js

@@ -200,7 +200,7 @@ async function assertNothingStagedOutside(dir) {
   if (outside) {
     throw bad(
       "ESTAGEDOUTSIDE",
-      `refusing to commit: ${outside} is staged outside this lane's directory`
+      `refusing to commit: ${outside} is staged outside this workspace's directory`
     );
   }
 }
@@ -505,7 +505,7 @@ async function mergeAction(dir, action, body) {
     if ((await git(dir, ["rev-parse", "--show-prefix"])).stdout.trim()) {
       throw bad(
         "ESUBDIR",
-        "a merge changes the whole repository; run it from a lane at the repo root"
+        "a merge changes the whole repository; run it from a workspace at the repo root"
       );
     }
     if (await isDirty(dir)) throw bad("EDIRTY", "working tree has uncommitted changes");