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)".

AGENTS.md

@@ -11,9 +11,9 @@
 - Update docs whenever workflow or architecture changes — follow `skills/update-project-docs/` automatically at the end of every change-set (README + VN/CN/KO mirrors, ARCHITECTURE, wiki + i18n + cache bump, server/client READMEs, docs/*).
 - Every applicable source file you create or update (`.js/.ts/.tsx/.cjs/.mjs/.py/.sh/.css`) must start with the authorship header: a truthful file overview plus the exact line `@author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>`. See `skills/file-headers/` and `.claude/rules/file-headers.md`; verify with `bash skills/file-headers/scripts/check-headers.sh`.
 
-## Lane stage
+## Workspace stage
 
-AgentHub's `codex` lanes (like every non-Claude AI tool lane) carry no pipeline: the dashboard shows no stage for
+AgentHub's `codex` workspaces — lanes in the API and CLI — (like every non-Claude AI tool workspace) carry no pipeline: the dashboard shows no stage for
 them and never infers or advances one, so there is no `agenthub stage` call to
 make while you work here as Codex.