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/mcp/targets.py

@@ -0,0 +1,16 @@
+"""Tab targeting for the MCP surface.
+
+MCP callers pay a full round trip for every extra tool call, so tools that act
+on a tab accept an optional ``tab_id`` and fall back to the browser's current
+active tab. Resolution happens here rather than by forwarding ``None`` into the
+SDK, so the acting tool always knows which tab it touched and can report it.
+"""
+from __future__ import annotations
+
+from browser_cli import BrowserCLI
+
+def resolve_tab_id(client: BrowserCLI, tab_id: int | None) -> int:
+  """Return *tab_id*, or the ID of the currently active tab when it is ``None``."""
+  if tab_id is not None:
+    return tab_id
+  return client.tabs.active().id