fix: auto-select extension with active targets when multiple Chrome profiles are connected

When multiple Chrome profiles have the playwriter extension installed, the
MCP fails with 'Multiple extensions connected. Specify extensionId.' even
if only one profile has playwriter-enabled tabs.

The relay's getExtensionConnection fallback now checks if exactly one
extension has active targets (connectedTargets > 0) and auto-selects it.
Falls through to the existing error when zero or multiple extensions are
active, so no regressions for single-extension setups.

The fix lives in the relay so all clients (MCP, Stagehand, raw CDP)
benefit. CLI session creation is unaffected — it still prompts the user
to pick a browser with --browser when multiple profiles are connected.

Fixes #52

playwriter/src/cdp-relay.ts

@@ -146,10 +146,26 @@ export async function startPlayWriterCDPRelayServer({
       return null
     }
 
-    const fallbackId = getDefaultExtensionId()
-    if (fallbackId) {
-      return extensionConnections.get(fallbackId) || null
+    // Single extension — use it directly
+    if (extensionConnections.size === 1) {
+      const fallbackId = getDefaultExtensionId()
+      if (fallbackId) {
+        return extensionConnections.get(fallbackId) || null
+      }
     }
+
+    // Multiple extensions — auto-select if exactly one has active targets.
+    // This handles the common case of multiple Chrome profiles with the extension
+    // installed, where only one profile has playwriter-enabled tabs. (#52)
+    if (extensionConnections.size > 1) {
+      const activeExtensions = Array.from(extensionConnections.values()).filter((ext) => {
+        return ext.connectedTargets.size > 0
+      })
+      if (activeExtensions.length === 1) {
+        return activeExtensions[0]
+      }
+    }
+
     return null
   }
 
@@ -838,10 +854,12 @@ export async function startPlayWriterCDPRelayServer({
       const clientId = c.req.param('clientId') || 'default'
       const url = new URL(c.req.url, 'http://localhost')
       const requestedExtensionId = url.searchParams.get('extensionId')
-      const resolvedExtension = getExtensionConnection(requestedExtensionId)
-      const allowDefault = !requestedExtensionId && extensionConnections.size === 1
-      const defaultExtension = allowDefault ? getExtensionConnection(null, { allowFallback: true }) : null
-      const clientExtensionId = resolvedExtension?.id || defaultExtension?.id || null
+      // When extensionId is explicit, resolve directly. Otherwise use fallback which
+      // handles single-extension and uniquely-active-extension cases (#52).
+      const resolvedExtension = requestedExtensionId
+        ? getExtensionConnection(requestedExtensionId)
+        : getExtensionConnection(null, { allowFallback: true })
+      const clientExtensionId = resolvedExtension?.id || null
 
       return {
         async onOpen(_event, ws) {