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>

docs/src/app/commands/page.mdx

@@ -223,8 +223,11 @@ The `frame` command accepts element refs (`@e3`), CSS selectors (`"#my-iframe"`)
 ```bash
 agent-browser dialog accept [text]    # Accept dialog (with optional prompt text)
 agent-browser dialog dismiss          # Dismiss dialog
+agent-browser dialog status           # Check if a dialog is currently open
 ```
 
+When a JavaScript dialog (`alert`, `confirm`, `prompt`) is pending, all command responses include a `warning` field with the dialog type and message.
+
 ## Debug
 
 ```bash