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/PLUGINS.md

@@ -93,7 +93,7 @@ Plugin skills are namespaced: they are invoked as `/agenthub:<name>`, with the b
 `/<name>` working as an alias unless another skill or command claims that name.
 This is why `skills/` sits at the repo root instead of `.claude/skills/` — the
 latter only loads for a session whose cwd is inside this checkout, which is
-useless for a lane working in a different repo.
+useless for a workspace working in a different repo.
 
 That is the whole plugin: the hooks, the CLI, the dashboard, three commands and
 four skills. It ships **no subagents** and no pipeline template, and it neither
@@ -260,7 +260,7 @@ On macOS the same steps write a LaunchAgent instead
 (`~/Library/LaunchAgents/io.smartgift.agenthub.plist`, `scripts/launchd-service.js`;
 `scripts/service-manager.js` picks the one for the platform, and both export the
 same API). `KeepAlive` plays `Restart=always`, `AbandonProcessGroup` keeps the
-lane panes' tmux server alive across a restart, output goes to
+workspace panes' tmux server alive across a restart, output goes to
 `~/Library/Logs/agenthub.log`, and the hand-over restart is `launchctl bootout`
 + `bootstrap` (a `kickstart` would keep the old plist). If that bootstrap fails
 and no server is left running, the bootstrap starts one detached. A LaunchAgent
@@ -272,7 +272,7 @@ server is a detached spawn started fresh on the next `SessionStart` after a
 reboot, same as always. That spawn inherits the Claude Code session's env, so
 `server/index.js` deletes the parent-session variables (`CLAUDECODE`,
 `CLAUDE_CODE_SESSION_ID`, `CLAUDE_CODE_MESSAGING_*`, `CLAUDE_PLUGIN_ROOT`, …)
-before anything else runs — otherwise tmux hands them to every lane pane. It is
+before anything else runs — otherwise tmux hands them to every workspace pane. It is
 a denylist: user configuration such as `CLAUDE_CONFIG_DIR`,
 `CLAUDE_CODE_USE_BEDROCK` and `ANTHROPIC_*` passes through. See
 [INSTALL.md § macOS](../INSTALL.md#macos).