feat(workspace): no double resume, server-picked tabs, terminal a11y

- A resume of a session a live pane already holds is refused (409
  ESESSIONLIVE with error.run_id) and the client opens that pane, pasting
  any typed message there; catalog tools refuse resumeSessionId
  (EUNSUPPORTED) instead of silently starting fresh.
- POST /api/lanes/:id/start takes slot: "auto": the server picks the free
  tab under the lane lock (components/lanes/startSession.ts on the client,
  kept out of WorkspaceHome so the sidebar does not pull react-markdown
  into the main bundle).
- Unsaved whole-file edits are guarded on every exit (Back, another file,
  tab switch, close, lane/session change, beforeunload).
- Closing an AI tab asks first; hiding a pane has its own icon. Tabs and
  pane headers show the session name, a state dot, rename by double-click.
- Terminal: Ctrl+M toggles Tab-moves-focus (no keyboard trap), Cmd+F /
  Ctrl+Shift+F find bar, clickable links (noopener), a write-only OSC 52
  clipboard, a Reconnecting overlay, debounced resizes, optional screen
  reader mode. Alt+1-4 / Alt+\ / Alt+W focus, split and hide panes.
- Changes panel: draggable splitter, wrapping git bar, history paging,
  disarming discard/undo/abort, inline diff comments pasted only into a
  visible pane; drag MIME guard on sidebar rows; IME-safe Enter; Show
  more in the sidebar and Recents; idle dots readable with sr-only text.
- Adds @xterm/addon-clipboard, @xterm/addon-search, @xterm/addon-web-links.

client/src/components/WorkspaceTree.tsx

@@ -168,11 +168,17 @@ export function WorkspaceTree() {
     }
   };
 
-  const onMenuAction = (lane: Lane, action: WorkspaceMenuAction) => {
+  const onMenuAction = (lane: Lane, action: WorkspaceMenuAction, runId?: string) => {
     if (action === "rename") setRenamingId(lane.id);
     else if (action === "new-here") openAdd(lane.source_repo || lane.cwd);
     else if (action === "reset" || action === "remove") setDestructive({ lane, action });
-    else if (action === "started") select(lane.id);
+    // Start opens the session it started, as the home's menu does.
+    else if (action === "started")
+      navigate(
+        runId
+          ? `/workspace?lane=${lane.id}&run=${encodeURIComponent(runId)}`
+          : `/workspace?lane=${lane.id}`
+      );
   };
 
   const confirmDestructive = async (body?: { expect?: unknown; force?: boolean }) => {
@@ -192,7 +198,8 @@ export function WorkspaceTree() {
     }
   };
 
-  const headBtn = "rounded p-1 text-fg-muted hover:bg-surface-3 hover:text-fg-primary";
+  // ≥24px targets (WCAG 2.5.8); each carries an aria-label.
+  const headBtn = "btn-icon";
   return (
     <section data-testid="workspace-tree" className="mt-3 border-t border-border pt-3">
       <div className="mb-1 flex items-center gap-1 px-2">
@@ -205,7 +212,7 @@ export function WorkspaceTree() {
             onChange={(e) => writeProjectTreePrefs({ tool: e.target.value || null })}
             aria-label={t("tree.toolFilter")}
             title={t("tree.toolFilter")}
-            className="max-w-[6.5rem] rounded border border-border bg-surface-1 px-1 py-0.5 text-[11px] text-fg-secondary"
+            className="select max-w-[6.5rem] !rounded !py-0.5 !pl-1 !pr-5 !text-[11px] text-fg-secondary"
           >
             <option value="">{t("tree.allTools")}</option>
             {tools.map((id) => (
@@ -307,7 +314,7 @@ export function WorkspaceTree() {
           x={menu.x}
           y={menu.y}
           onClose={() => setMenu(null)}
-          onAction={(a) => onMenuAction(menu.lane, a)}
+          onAction={(a, runId) => onMenuAction(menu.lane, a, runId)}
         />
       )}
       {destructive && (