Default MCP tab tools to the active tab

Requiring an explicit tab_id forced every navigate or close through a
preceding tabs_list call, which costs an MCP client a full round trip just to
learn the ID the browser already considers current.

navigate and tabs_close now resolve the active tab when tab_id is omitted,
matching the screenshot tool. Resolution is explicit rather than forwarding
None into the SDK, so the acting tool knows which tab it touched; tabs_close
reports it, since closing the wrong tab is not recoverable.

This stays in the MCP layer: the SDK and CLI signatures are unchanged.

browser_cli/remote/pool.py

@@ -26,6 +26,7 @@ from browser_cli.framing import frame
 # hand back one the server has just timed out and closed.
 _MAX_IDLE_SECONDS = max(5, REMOTE_SESSION_IDLE_TIMEOUT - 5)
 _MAX_PER_ENDPOINT = 8
+_MAX_ENDPOINTS = 64
 
 class PooledConnection:
   __slots__ = ("sock", "secret", "last_used")
@@ -56,10 +57,32 @@ def checkout(endpoint: str) -> PooledConnection | None:
       _close(conn.sock)  # too old — assume the server has dropped it
   return None
 
+def _prune_endpoints_locked(now: float) -> None:
+  """Keep the number of endpoint buckets bounded for long-running SDK users."""
+  for endpoint, bucket in list(_POOL.items()):
+    fresh = [conn for conn in bucket if now - conn.last_used <= _MAX_IDLE_SECONDS]
+    if fresh:
+      _POOL[endpoint] = fresh
+    else:
+      for conn in bucket:
+        _close(conn.sock)
+      _POOL.pop(endpoint, None)
+
+  while len(_POOL) >= _MAX_ENDPOINTS:
+    oldest_endpoint, bucket = min(
+      _POOL.items(),
+      key=lambda item: min(conn.last_used for conn in item[1]) if item[1] else 0.0,
+    )
+    for conn in bucket:
+      _close(conn.sock)
+    _POOL.pop(oldest_endpoint, None)
+
 def checkin(endpoint: str, conn: PooledConnection) -> None:
   """Return a still-healthy connection to the pool for reuse."""
   conn.last_used = time.monotonic()
   with _LOCK:
+    if endpoint not in _POOL and len(_POOL) >= _MAX_ENDPOINTS:
+      _prune_endpoints_locked(conn.last_used)
     bucket = _POOL.setdefault(endpoint, [])
     if len(bucket) >= _MAX_PER_ENDPOINT:
       _close(conn.sock)