feat: sync agent-browser skill with upstream vercel-labs/agent-browser

Update SKILL.md to match the latest upstream skill from
vercel-labs/agent-browser, adding substantial new capabilities:

- Authentication (auth vault, profiles, session persistence, state files)
- Command chaining, annotated screenshots, diffing
- Security features (content boundaries, domain allowlist, action policy)
- iOS Simulator support, Lightpanda engine, downloads, clipboard
- JS eval improvements (--stdin, -b for shell safety)
- Timeout guidance, config files, session cleanup

Add 7 reference docs (commands, authentication, snapshot-refs,
session-management, video-recording, profiling, proxy-support) and
3 ready-to-use shell templates.

Kept our YAML frontmatter, setup check section, and Playwright MCP
comparison table which are unique to our plugin context.

plugins/compound-engineering/skills/agent-browser/templates/authenticated-session.sh

@@ -0,0 +1,105 @@
+#!/bin/bash
+# Template: Authenticated Session Workflow
+# Purpose: Login once, save state, reuse for subsequent runs
+# Usage: ./authenticated-session.sh <login-url> [state-file]
+#
+# RECOMMENDED: Use the auth vault instead of this template:
+#   echo "<pass>" | agent-browser auth save myapp --url <login-url> --username <user> --password-stdin
+#   agent-browser auth login myapp
+# The auth vault stores credentials securely and the LLM never sees passwords.
+#
+# Environment variables:
+#   APP_USERNAME - Login username/email
+#   APP_PASSWORD - Login password
+#
+# Two modes:
+#   1. Discovery mode (default): Shows form structure so you can identify refs
+#   2. Login mode: Performs actual login after you update the refs
+#
+# Setup steps:
+#   1. Run once to see form structure (discovery mode)
+#   2. Update refs in LOGIN FLOW section below
+#   3. Set APP_USERNAME and APP_PASSWORD
+#   4. Delete the DISCOVERY section
+
+set -euo pipefail
+
+LOGIN_URL="${1:?Usage: $0 <login-url> [state-file]}"
+STATE_FILE="${2:-./auth-state.json}"
+
+echo "Authentication workflow: $LOGIN_URL"
+
+# ================================================================
+# SAVED STATE: Skip login if valid saved state exists
+# ================================================================
+if [[ -f "$STATE_FILE" ]]; then
+    echo "Loading saved state from $STATE_FILE..."
+    if agent-browser --state "$STATE_FILE" open "$LOGIN_URL" 2>/dev/null; then
+        agent-browser wait --load networkidle
+
+        CURRENT_URL=$(agent-browser get url)
+        if [[ "$CURRENT_URL" != *"login"* ]] && [[ "$CURRENT_URL" != *"signin"* ]]; then
+            echo "Session restored successfully"
+            agent-browser snapshot -i
+            exit 0
+        fi
+        echo "Session expired, performing fresh login..."
+        agent-browser close 2>/dev/null || true
+    else
+        echo "Failed to load state, re-authenticating..."
+    fi
+    rm -f "$STATE_FILE"
+fi
+
+# ================================================================
+# DISCOVERY MODE: Shows form structure (delete after setup)
+# ================================================================
+echo "Opening login page..."
+agent-browser open "$LOGIN_URL"
+agent-browser wait --load networkidle
+
+echo ""
+echo "Login form structure:"
+echo "---"
+agent-browser snapshot -i
+echo "---"
+echo ""
+echo "Next steps:"
+echo "  1. Note the refs: username=@e?, password=@e?, submit=@e?"
+echo "  2. Update the LOGIN FLOW section below with your refs"
+echo "  3. Set: export APP_USERNAME='...' APP_PASSWORD='...'"
+echo "  4. Delete this DISCOVERY MODE section"
+echo ""
+agent-browser close
+exit 0
+
+# ================================================================
+# LOGIN FLOW: Uncomment and customize after discovery
+# ================================================================
+# : "${APP_USERNAME:?Set APP_USERNAME environment variable}"
+# : "${APP_PASSWORD:?Set APP_PASSWORD environment variable}"
+#
+# agent-browser open "$LOGIN_URL"
+# agent-browser wait --load networkidle
+# agent-browser snapshot -i
+#
+# # Fill credentials (update refs to match your form)
+# agent-browser fill @e1 "$APP_USERNAME"
+# agent-browser fill @e2 "$APP_PASSWORD"
+# agent-browser click @e3
+# agent-browser wait --load networkidle
+#
+# # Verify login succeeded
+# FINAL_URL=$(agent-browser get url)
+# if [[ "$FINAL_URL" == *"login"* ]] || [[ "$FINAL_URL" == *"signin"* ]]; then
+#     echo "Login failed - still on login page"
+#     agent-browser screenshot /tmp/login-failed.png
+#     agent-browser close
+#     exit 1
+# fi
+#
+# # Save state for future runs
+# echo "Saving state to $STATE_FILE"
+# agent-browser state save "$STATE_FILE"
+# echo "Login successful"
+# agent-browser snapshot -i