feat: add dialog detection and document dialog commands (#999)

Fixes #992

When a JavaScript dialog (alert/confirm/prompt) blocks the page, agents
had no way to detect it — all commands just timed out with generic errors.

- Add `dialog status` command to check for pending dialogs
- Track dialog state via CDP Page.javascriptDialogOpening/Closed events
- Auto-inject `warning` field into all command responses when a dialog is
  pending, so agents can distinguish dialog-blocked timeouts from other issues
- Document dialog commands in SKILL.md (was missing entirely), README.md,
  docs site, and --help output

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

README.md

@@ -300,8 +300,11 @@ agent-browser frame main              # Back to main frame
 ```bash
 agent-browser dialog accept [text]    # Accept (with optional prompt text)
 agent-browser dialog dismiss          # Dismiss
+agent-browser dialog status           # Check if a dialog is currently open
 ```
 
+When a JavaScript dialog is pending, all command responses include a `warning` field with the dialog type and message.
+
 ### Diff
 
 ```bash