Add auto-dismissal for alert and beforeunload dialogs (#1075)

* Add auto-dismissal for alert and beforeunload dialogs

This PR adds automatic handling of JavaScript dialogs to prevent the agent from blocking indefinitely when `alert()` or `beforeunload` dialogs appear on web pages.

## Summary

Previously, when a website displayed native browser confirmation dialogs (like alerts or "Are you sure you want to leave?" prompts), agent-browser would hang waiting for manual intervention. This is a common issue since many websites use these dialogs for notifications or navigation warnings.

## Changes Made

- **Auto-dismiss functionality**: Added a background task that automatically accepts `alert` and `beforeunload` dialogs while leaving `confirm` and `prompt` dialogs for explicit handling
- **New flag**: Added `--no-auto-dialog` flag to disable automatic handling when needed
- **Environment variable**: Added `AGENT_BROWSER_NO_AUTO_DIALOG` for configuration
- **Documentation**: Updated README and docs with usage examples and configuration details
- **Tests**: Added comprehensive test coverage for flag parsing and dialog handling logic

## Implementation Details

- Only `alert` (notification-only) and `beforeunload` (navigation warning) dialogs are auto-handled for safety
- `confirm` and `prompt` dialogs still require explicit `dialog accept/dismiss` commands to ensure agents make deliberate choices for destructive actions
- The feature is enabled by default since these dialog types rarely require user decision-making
- Uses Chrome DevTools Protocol's `Page.handleJavaScriptDialog` for reliable dialog dismissal

Fixes #1070

* Log dialog type and message before auto-dismissal

Without this, auto-dismissed alert/beforeunload dialogs are silently
swallowed and the agent has no way to see what the dialog said. Adding
an eprintln before the CDP call makes the dismissal visible in stderr
for debugging.

* Log dialog dismissal errors instead of silently discarding them

- Remove premature "accepted" from log message since it fires before
  the CDP command executes
- Replace `let _ =` with `if let Err(e)` to log failures when
  Page.handleJavaScriptDialog fails
- Apply rustfmt to auto-dialog tests

---------

Co-authored-by: ctate <366502+ctate@users.noreply.github.com>

skills/agent-browser/SKILL.md

@@ -184,7 +184,10 @@ agent-browser clipboard write "Hello, World!"     # Write text to clipboard
 agent-browser clipboard copy                      # Copy current selection
 agent-browser clipboard paste                     # Paste from clipboard
 
-# Dialogs (alert, confirm, prompt)
+# Dialogs (alert, confirm, prompt, beforeunload)
+# By default, alert and beforeunload dialogs are auto-accepted so they never block the agent.
+# confirm and prompt dialogs still require explicit handling.
+# Use --no-auto-dialog to disable automatic handling.
 agent-browser dialog accept              # Accept dialog
 agent-browser dialog accept "my input"   # Accept prompt dialog with text
 agent-browser dialog dismiss             # Dismiss/cancel dialog