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/components/Workspace.tsx

@@ -65,6 +65,8 @@ export function Workspace({
     showLandingPage,
     addSplitPane,
     removePane,
+    reopenPane,
+    hasClosedPanes,
     isPaneChatPending,
     handlePaneDragStart,
     handlePaneDragOver,
@@ -207,10 +209,9 @@ export function Workspace({
                   onCloseOthers={(chatId) => closeOtherTabs(idx, chatId)}
                   onCloseToRight={(chatId) => closeTabsToRight(idx, chatId)}
                   onCloseAll={() => closeAllTabs(idx)}
-                  onAddPane={(kind) => {
-                    if (kind === 'chat') void createChat(idx);
-                    else onAddPane(kind);
-                  }}
+                  onNewTab={() => void createChat(idx)}
+                  onSplitPane={(kind) => onAddPane(kind)}
+                  onReopenPane={hasClosedPanes ? reopenPane : undefined}
                   onShowHistory={() => showLandingPage(idx)}
                   onRename={renameChat}
                   onRemovePane={panes.length > 1 ? () => removePane(idx) : undefined}