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/server.py

@@ -16,6 +16,7 @@ from urllib.parse import urlsplit
 from browser_cli import BrowserCLI
 from browser_cli.mcp.naming import resolve_tool_prefix, tool_name
 from browser_cli.mcp.serialization import structured
+from browser_cli.mcp.targets import resolve_tab_id
 
 ClientFactory = Callable[..., BrowserCLI]
 
@@ -93,27 +94,29 @@ def create_server(*, client_factory: ClientFactory = BrowserCLI, tool_prefix: st
 
   @mcp.tool(name=tool_name("tabs_close", prefix))
   def tabs_close(
-    tab_id: int,
+    tab_id: int | None = None,
     browser: str | None = None,
     remote: str | None = None,
     key: str | None = None,
   ) -> dict[str, int]:
-    """Close one tab by its current ID. This changes the user's real browser."""
-    closed = _client(client_factory, browser, remote, key).tabs.close(tab_id)
-    return {"closed": closed}
+    """Close a tab, defaulting to the active tab. This changes the real browser."""
+    client = _client(client_factory, browser, remote, key)
+    target = resolve_tab_id(client, tab_id)
+    return {"closed": client.tabs.close(target), "tab_id": target}
 
   @mcp.tool(name=tool_name("navigate", prefix))
   def navigate(
-    tab_id: int,
     url: str,
+    tab_id: int | None = None,
     browser: str | None = None,
     remote: str | None = None,
     key: str | None = None,
   ) -> dict[str, Any]:
-    """Navigate an existing tab to a URL and return a fresh tab snapshot."""
+    """Navigate a tab to a URL, defaulting to the active tab, and return it."""
     client = _client(client_factory, browser, remote, key)
-    client.nav.to(tab_id, url)
-    return structured(client.tabs.status(tab_id))
+    target = resolve_tab_id(client, tab_id)
+    client.nav.to(target, url)
+    return structured(client.tabs.status(target))
 
   @mcp.tool(name=tool_name("page_info", prefix))
   def page_info(