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-sync.js

@@ -282,7 +282,7 @@ async function continueSync(lane, profile, branchArg) {
 
   const current = (await git(lane.cwd, ["rev-parse", "--abbrev-ref", "HEAD"])).stdout.trim();
   if (current !== branch) {
-    throw badBranch(`--continue: lane is not on '${branch}' (currently on '${current}')`);
+    throw badBranch(`--continue: workspace is not on '${branch}' (currently on '${current}')`);
   }
 
   const unresolved = await unmergedFiles(lane.cwd);