feat: strip dcp-message-id tags from opencode output + reopen closed panes

Two independent fixes:

- opencode-server.ts: stripDcpTags() removes <dcp-message-id>…</dcp-message-id>
  tags from text deltas before they reach the frame/DB. Applied to all three
  text paths (session.next.text.delta, message.part.delta text field,
  handleUpdatedPart text type). Reasoning/tool paths untouched.
- useWorkspacePanes.ts: module-level closedPaneStack (capped at 10) captures
  pane kind + chatIds on removePane and removeTab auto-remove. reopenPane()
  pops the stack and re-attaches a new pane to the existing chat ids (chats
  survive pane close server-side). hasClosedPanes drives conditional render.
- ChatTabBar.tsx: [+] is now instant new-tab (no dropdown); split-pane
  dropdown (Columns2 icon) opens Chat/Term/Code in a new pane; reopen button
  (RotateCcw icon) appears when closed panes exist.
- Workspace.tsx: pass reopenPane + hasClosedPanes through to ChatTabBar.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/useWorkspacePanes.ts

@@ -32,6 +32,21 @@ function chatPane(chatId: string): WorkspacePane {
   return { id: generateId(), kind: 'chat', chatId, chatIds: [chatId], activeChatIdx: 0 };
 }
 
+interface ClosedPaneEntry {
+  kind: WorkspacePane['kind'];
+  chatIds: string[];
+  activeChatIdx: number;
+}
+const MAX_CLOSED = 10;
+const closedPaneStack: ClosedPaneEntry[] = [];
+
+function pushClosed(pane: WorkspacePane): void {
+  if (pane.kind === 'empty' || pane.kind === 'settings') return;
+  if (pane.chatIds.length === 0) return;
+  closedPaneStack.push({ kind: pane.kind, chatIds: [...pane.chatIds], activeChatIdx: pane.activeChatIdx });
+  if (closedPaneStack.length > MAX_CLOSED) closedPaneStack.shift();
+}
+
 function chatNameForPaneKind(kind: 'coder' | 'terminal'): string {
   return kind === 'coder' ? 'BooCoder' : 'Terminal';
 }
@@ -137,6 +152,8 @@ export interface UseWorkspacePanesResult {
   // falls back to an empty pane to preserve the "always one pane" invariant.
   toggleSettingsPane: () => string | null;
   removePane: (idx: number) => void;
+  reopenPane: () => void;
+  hasClosedPanes: boolean;
   removeChatFromPanes: (chatId: string) => void;
   initializeFirstChatIfEmpty: (chatId: string) => void;
   validatePanes: (validChatIds: Set<string>) => void;
@@ -394,6 +411,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
         if (next.length > 1) {
           // Last tab closed and other panes exist — remove the whole pane
           // instead of leaving an orphaned empty panel.
+          pushClosed(pane); setHasClosedPanes(true);
           const spliced = next.filter((_, i) => i !== paneIdx);
           setActivePaneIdx((ai) => Math.min(ai, spliced.length - 1));
           return spliced;
@@ -541,6 +559,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
       // The endpoint is idempotent (404 on missing session) so a strict-mode
       // double-invoke of the updater is safe.
       const removed = prev[idx];
+      if (removed) { pushClosed(removed); setHasClosedPanes(true); }
       if (removed?.kind === 'terminal') {
         api.terminals.kill(sessionId, removed.id).catch(() => { /* non-fatal */ });
       }
@@ -550,6 +569,26 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     });
   }, [sessionId]);
 
+  const [hasClosedPanes, setHasClosedPanes] = useState(closedPaneStack.length > 0);
+
+  const reopenPane = useCallback(() => {
+    const entry = closedPaneStack.pop();
+    setHasClosedPanes(closedPaneStack.length > 0);
+    if (!entry) return;
+    setPanes((prev) => {
+      const restored: WorkspacePane = {
+        id: generateId(),
+        kind: entry.kind,
+        chatId: entry.chatIds[entry.activeChatIdx] ?? entry.chatIds[0],
+        chatIds: entry.chatIds,
+        activeChatIdx: Math.min(entry.activeChatIdx, entry.chatIds.length - 1),
+      };
+      const next = [...prev, restored];
+      setActivePaneIdx(next.length - 1);
+      return next;
+    });
+  }, []);
+
   // Replaces a single empty default pane with a chat pane. Used by the initial
   // chat fetch to land on the most-recent open chat if no saved pane state.
   const initializeFirstChatIfEmpty = useCallback((chatId: string) => {
@@ -679,6 +718,8 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     addSplitPane,
     toggleSettingsPane,
     removePane,
+    reopenPane,
+    hasClosedPanes,
     removeChatFromPanes,
     initializeFirstChatIfEmpty,
     validatePanes,