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/coder/src/services/backends/opencode-server.ts

@@ -192,7 +192,8 @@ export class OpenCodeServerBackend implements AgentBackend {
         const st = this.byOpencodeId.get(p.sessionID);
         if (!st?.activeTurn) return;
         this.bumpActivity(st);
-        st.activeTurn.onEvent({ type: 'text', text: p.delta });
+        const cleaned = stripDcpTags(p.delta);
+        if (cleaned) st.activeTurn.onEvent({ type: 'text', text: cleaned });
         return;
       }
       case 'session.next.reasoning.delta': {
@@ -264,7 +265,8 @@ export class OpenCodeServerBackend implements AgentBackend {
           st.activeTurn.onEvent({ type: 'reasoning', text: p.delta });
         } else if (p.field === 'text') {
           st.streamedPartKeys.add(`text:${p.partID}`);
-          st.activeTurn.onEvent({ type: 'text', text: p.delta });
+          const cleaned = stripDcpTags(p.delta);
+          if (cleaned) st.activeTurn.onEvent({ type: 'text', text: cleaned });
         }
         return;
       }
@@ -301,7 +303,8 @@ export class OpenCodeServerBackend implements AgentBackend {
       st.partTypeById.set(part.id, part.type);
       const key = resolvePartDedupeKey(part, part.type);
       if (key && st.streamedPartKeys.delete(key)) return; // already streamed via delta
-      const text = part.text ?? '';
+      const raw = part.text ?? '';
+      const text = part.type === 'text' ? stripDcpTags(raw) : raw;
       if (text && part.time?.end != null) {
         turn.onEvent({ type: part.type, text });
       }
@@ -717,6 +720,11 @@ function sleep(ms: number): Promise<void> {
   return new Promise((r) => setTimeout(r, ms));
 }
 
+/** Strip opencode-dcp plugin tags that render as literal text in the UI. */
+function stripDcpTags(s: string): string {
+  return s.replace(/<dcp-message-id>[^<]*<\/dcp-message-id>/g, '');
+}
+
 function errMsg(e: unknown): string {
   return e instanceof Error ? e.message : String(e);
 }