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.

README.md

@@ -174,6 +174,11 @@ The server is stateless at the MCP layer. Every tool call creates a fresh
 `browser`, `remote`, and `key` on a tool call when a specific local profile or
 authenticated browser-cli remote is required.
 
+`browser_navigate`, `browser_tabs_close`, and `browser_screenshot` take an
+optional `tab_id` and act on the active tab when it is omitted, so a caller
+does not need a preceding `browser_tabs_list` round trip. `browser_tabs_close`
+reports the tab it closed.
+
 Available tools:
 - `browser_tabs_list`, `browser_tabs_open`, `browser_tabs_close`
 - `browser_navigate`, `browser_page_info`