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>

skills/agent-browser/SKILL.md

@@ -177,6 +177,12 @@ 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)
+agent-browser dialog accept              # Accept dialog
+agent-browser dialog accept "my input"   # Accept prompt dialog with text
+agent-browser dialog dismiss             # Dismiss/cancel dialog
+agent-browser dialog status              # Check if a dialog is currently open
+
 # Diff (compare page states)
 agent-browser diff snapshot                          # Compare current vs last snapshot
 agent-browser diff snapshot --baseline before.txt    # Compare current vs saved file
@@ -522,6 +528,26 @@ agent-browser wait 5000
 
 When dealing with consistently slow websites, use `wait --load networkidle` after `open` to ensure the page is fully loaded before taking a snapshot. If a specific element is slow to render, wait for it directly with `wait <selector>` or `wait @ref`.
 
+## JavaScript Dialogs (alert / confirm / prompt)
+
+When a page opens a JavaScript dialog (`alert()`, `confirm()`, or `prompt()`), it blocks all other browser commands (snapshot, screenshot, click, etc.) until the dialog is dismissed. If commands start timing out unexpectedly, check for a pending dialog:
+
+```bash
+# Check if a dialog is blocking
+agent-browser dialog status
+
+# Accept the dialog (dismiss the alert / click OK)
+agent-browser dialog accept
+
+# Accept a prompt dialog with input text
+agent-browser dialog accept "my input"
+
+# Dismiss the dialog (click Cancel)
+agent-browser dialog dismiss
+```
+
+When a dialog is pending, all command responses include a `warning` field indicating the dialog type and message. In `--json` mode this appears as a `"warning"` key in the response object.
+
 ## Session Management and Cleanup
 
 When running multiple agents or automations concurrently, always use named sessions to avoid conflicts: