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)".

docs/HOOKS.md

@@ -586,8 +586,8 @@ function sendToServer(hookType, payload) {
 >    entry whose PID is no longer alive** and POSTs the hook payload to one
 >    port per **unique SQLite data directory** (lowest port wins when two
 >    instances share `~/.claude/agenthub`). The other instance re-broadcasts
->    the lanes those hooks moved within `DASHBOARD_LANE_SYNC_MS` (2 s), so its
->    browsers see a lane's status without a refresh.
+>    the workspaces those hooks moved within `DASHBOARD_LANE_SYNC_MS` (2 s), so its
+>    browsers see a workspace's status without a refresh.
 > 3. If neither yields a target, the handler falls back to `4820`.
 >
 > Dashboards with **different** databases (e.g. the packaged desktop app using