docs(skills): document console-capture opt-in + stealth env knobs

console/errors capture is off by default in this fork (Runtime.enable is a
detectable CDP signal). Update the agent-facing skill docs so agents don't
treat empty console output as a bug:

- commands.md: new "Stealth / anti-detection knobs" env-var block
  (CAPTURE_CONSOLE, TIMEZONE, BLOCK_WEBRTC, HIDE_CANVAS, ADAPTIVE_REF) plus a
  heads-up note; annotate the console/errors lines.
- dogfood/slack SKILL.md: note that console/errors need
  AGENT_BROWSER_CAPTURE_CONSOLE=1.

skill-data/slack/SKILL.md

@@ -230,6 +230,9 @@ agent-browser snapshot -i | grep -c "treeitem"
 
 ### Check console for errors
 
+Console/error capture is off by default in this stealth fork — launch the session with
+`AGENT_BROWSER_CAPTURE_CONSOLE=1` first, or these return empty.
+
 ```bash
 agent-browser console
 agent-browser errors