fix(bridge): use OS-assigned port for auto-started web servers

tryStartWebServer used findFreePort to walk from coordinatorPort+1
(19877), which has a TOCTOU race — it probes, closes the probe, then
the caller binds. Between close and bind, a concurrent agent grabs the
same port. This caused EADDRINUSE when parallel subagents all started
web servers simultaneously.

Port 0 eliminates the race: the OS atomically assigns a unique free
port for each listener. The PWA discovery path (probe from 19877) is
only used when the page is served from a non-local host (e.g. GitHub
Pages). When served locally (the bridge-started case), the browser
connects via location.host directly — the port number doesn't need to
be predictable.

Remove the coordinatorPort parameter from tryStartWebServer — it was
only used to compute the scan base, which is no longer needed.

Author: Mearman

src/bridges/pi/index.ts

@@ -146,7 +146,7 @@ export default function (pi: ExtensionAPI) {
         cwd: process.cwd(),
         pid: process.pid,
       });
-      webHandle = await tryStartWebServer(webCtrl, store.coordinatorPort);
+      webHandle = await tryStartWebServer(webCtrl);
     }
 
     refreshStatus();

src/bridges/user/web/server.ts

@@ -29,7 +29,6 @@ import type {
   MeshMessage,
   MeshStatePatch,
 } from "../../../core/wire-protocol.js";
-import { findFreePort } from "./port-discovery.js";
 
 const WEB_HOST = "127.0.0.1";
 
@@ -101,17 +100,21 @@ export interface WebServerHandle {
 // ---------------------------------------------------------------------------
 
 /**
- * Start the web UI server on an OS-assigned port.
- * Returns the server handle, or undefined if port discovery fails.
+ * Start the web UI server on an OS-assigned free port.
+ *
+ * Uses port 0 (OS-assigned) to avoid TOCTOU races when multiple bridges
+ * start web servers concurrently — the OS atomically allocates a unique
+ * free port for each.
+ *
+ * The PWA discovery path (probe from 19877) is only used when the page is
+ * served from a non-local host (e.g. GitHub Pages). When served locally
+ * (the common case for bridge-started servers), the browser connects via
+ * location.host directly — so the port number doesn't need to be predictable.
  */
 export async function tryStartWebServer(
   controller?: ChatController,
-  coordinatorPort?: number,
 ): Promise<WebServerHandle | undefined> {
-  const base = (coordinatorPort ?? 19876) + 1;
-  const port = await findFreePort(base);
-  if (port === undefined) return undefined;
-  return createWebServer(port, controller);
+  return createWebServer(0, controller);
 }
 
 /**