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/lanes.js

@@ -480,7 +480,7 @@ function appendStageEvent(lane, nodeId, { source, result, evidence } = {}) {
 
 function setStage(id, { stage, status, evidence, note, result } = {}) {
   const lane = getLane(id);
-  if (!lane) throw Object.assign(new Error(`no lane ${id}`), { code: "ENOLANE" });
+  if (!lane) throw Object.assign(new Error(`no workspace ${id}`), { code: "ENOLANE" });
   const next = stage || lane.stage;
   const changed = next !== lane.stage;
   const stages = { ...lane.stages };
@@ -543,7 +543,7 @@ function setStage(id, { stage, status, evidence, note, result } = {}) {
  */
 function recordDetection(id, { nodeId, signal } = {}) {
   const lane = getLane(id);
-  if (!lane) throw Object.assign(new Error(`no lane ${id}`), { code: "ENOLANE" });
+  if (!lane) throw Object.assign(new Error(`no workspace ${id}`), { code: "ENOLANE" });
   const pipeline = getPipeline(lane.pipeline);
   const nodeIdx = phaseIdx(pipeline, nodeId);
   if (nodeIdx === -1) return { written: false, reason: "unknown-node" };
@@ -839,7 +839,7 @@ function hasActiveLaneSession(lane) {
  */
 function purgeLaneSessions(laneId) {
   const lane = getLane(laneId);
-  if (!lane) throw Object.assign(new Error(`no lane ${laneId}`), { code: "ENOLANE" });
+  if (!lane) throw Object.assign(new Error(`no workspace ${laneId}`), { code: "ENOLANE" });
 
   const result = { sessions: 0, events: 0, tokenRows: 0 };