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)".

plugins/agenthub/commands/agenthub-doctor.md

@@ -31,7 +31,7 @@ Print the output verbatim, then add at most three lines of guidance based on the
   output as the immediate fix. "installed but not running" just needs
   `systemctl --user start agenthub`.
 - `WARN tmux` → only the run console, `agenthub lanes shell` and the
-  session-reload keystrokes are affected; monitoring, lanes and the CLI are not.
+  session-reload keystrokes are affected; monitoring, workspaces and the CLI are not.
   Print the install command from the output. If the row says `ok` but Start Run
   still fails, the server has a different PATH than this check — ask it directly
   with `curl -s localhost:4820/api/run/tmux`.