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>

cli/src/connection.rs

@@ -26,6 +26,8 @@ pub struct Response {
     pub success: bool,
     pub data: Option<Value>,
     pub error: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub warning: Option<String>,
 }
 
 #[allow(dead_code)]