add --selector flag to scroll command (#537)

* add --selector flag to scroll command

The `scroll` command uses `window.scrollBy()`, which has no effect on apps
that use custom scrollable containers (e.g. a nested div with overflow-y: auto).

The backend `handleScroll` already supports a `selector` parameter, but the CLI
never exposed it. This adds `-s` / `--selector` to the `scroll` command so users
can target a specific scrollable element:

    agent-browser scroll down 500 --selector "div.scroll-container"

Also fixes the backend to apply `direction`/`amount` when a selector is present
(previously those fields were only used in the no-selector branch).

Closes #501

* fixes

cli/src/output.rs

@@ -973,14 +973,17 @@ Use Cases:
             r##"
 agent-browser scroll - Scroll the page
 
-Usage: agent-browser scroll [direction] [amount]
+Usage: agent-browser scroll [direction] [amount] [options]
 
-Scrolls the page in the specified direction.
+Scrolls the page or a specific element in the specified direction.
 
 Arguments:
   direction            up, down, left, right (default: down)
   amount               Pixels to scroll (default: 300)
 
+Options:
+  -s, --selector <sel> CSS selector for a scrollable container
+
 Global Options:
   --json               Output as JSON
   --session <name>     Use specific session
@@ -990,6 +993,7 @@ Examples:
   agent-browser scroll down 500
   agent-browser scroll up 200
   agent-browser scroll left 100
+  agent-browser scroll down 500 --selector "div.scroll-container"
 "##
         }
         "scrollintoview" | "scrollinto" => {