server, ui: only offer a working directory when a tool reads it (#26762)

The working directory chip showed up as soon as the server exposed any
builtin tool, so a server started with just get_datetime, or a user who
turned every filesystem tool off in the settings, still got a control
that nothing would read.

Tools now declare whether they resolve their paths and run against the
working directory, next to the write permission they already publish in
the /tools listing. The WebUI shows the chip and enables the /cwd
command only when at least one such tool is both served and left
enabled.

tools/server/server-tools.h

@@ -14,6 +14,7 @@ struct server_tool {
     std::string display_name;
     bool permission_write = false;
     bool support_stream = false; // if true, output can be streamed
+    bool uses_cwd = false;       // if true, the tool resolves paths and runs against the working directory
 
     virtual ~server_tool() = default;
     virtual json get_definition() const = 0;