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

skills/agent-browser/SKILL.md

@@ -67,6 +67,7 @@ agent-browser press Enter             # Press key
 agent-browser keyboard type "text"    # Type at current focus (no selector)
 agent-browser keyboard inserttext "text"  # Insert without key events
 agent-browser scroll down 500         # Scroll page
+agent-browser scroll down 500 --selector "div.content"  # Scroll within a specific container
 
 # Get information
 agent-browser get text @e1            # Get element text