revert the chat tabs and panes work, back to one conversation

Andre asked for zero, not another fix on top. Reverts ec4f06a..7726c9f — the ten commits from
"tabs and panes" onward: the tab bar and pane splitting, tab renaming and its page title, the
per-server directory picker, the render-loop fix, pane transcript resolution, the send queue,
the two socket fixes from the other session, the pane-socket notes, and my own socket-set change
from tonight. He is rebuilding from here.

Deliberately KEPT: dc6b623, "talk to two officers at once from one browser". That was a separate
ask that predates the tabs one, and the multi-server client, the server chips and the connections
store stand on their own without panes. Reverting it too is one more command if that was the
intent.

Collateral, worth naming: cb7ab55 carried an unrelated MusicPlayerHost change alongside its
socket instrumentation, so that came out with it.

Reverts, not a reset — every one of these is pushed and a second session is live in this repo.

Typecheck clean. 600 pass, 2 fail — cliamp path-escape and the pty transport test, both failing
identically before this and unrelated to chat.

What is NOT explained by this revert: the browser symptoms tonight. The server was verified good
throughout — two real turns streamed back through the public URL on both models, and the full
2,281-message history came through nginx intact. Whatever the client fault is, it is still
unfound, and the pre-tabs code is where it now has to be looked for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/api/chat/session-manager.ts

@@ -24,7 +24,7 @@ class SessionManager {
         cwd,
         model,
         piProcess: null,
-        sockets: new Set(),
+        ws: null,
         lastActivity: Date.now(),
         idleTimer: null,
         streamBuffer: '',
@@ -145,7 +145,7 @@ class SessionManager {
   attachWs(sessionId: string, ws: any): void {
     const session = this.sessions.get(sessionId);
     if (session) {
-      session.sockets.add(ws);
+      session.ws = ws;
       session.lastActivity = Date.now();
 
       if (session.idleTimer) {
@@ -155,24 +155,14 @@ class SessionManager {
     }
   }
 
-  /**
-   * Removes one socket. The caller must say WHICH — a bare `detachWs(sessionId)` used to null the
-   * session's only socket field, so a stale client's close event silenced whichever client had attached
-   * after it. A close is only the end of the conversation when nothing else is still watching.
-   */
-  detachWs(sessionId: string, ws: any): void {
+  detachWs(sessionId: string): void {
     const session = this.sessions.get(sessionId);
     if (session) {
-      session.sockets.delete(ws);
+      session.ws = null;
       session.lastActivity = Date.now();
     }
   }
 
-  /** Whether anything is still watching — the idle GC must not start while another client is attached. */
-  hasSockets(sessionId: string): boolean {
-    return (this.sessions.get(sessionId)?.sockets.size ?? 0) > 0;
-  }
-
   setIdleTimeout(sessionId: string, timeoutMs: number): void {
     const session = this.sessions.get(sessionId);
     if (!session) return;