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/references/session-management.md

@@ -0,0 +1,193 @@
+# Session Management
+
+Multiple isolated browser sessions with state persistence and concurrent browsing.
+
+**Related**: [authentication.md](authentication.md) for login patterns, [SKILL.md](../SKILL.md) for quick start.
+
+## Contents
+
+- [Named Sessions](#named-sessions)
+- [Session Isolation Properties](#session-isolation-properties)
+- [Session State Persistence](#session-state-persistence)
+- [Common Patterns](#common-patterns)
+- [Default Session](#default-session)
+- [Session Cleanup](#session-cleanup)
+- [Best Practices](#best-practices)
+
+## Named Sessions
+
+Use `--session` flag to isolate browser contexts:
+
+```bash
+# Session 1: Authentication flow
+agent-browser --session auth open https://app.example.com/login
+
+# Session 2: Public browsing (separate cookies, storage)
+agent-browser --session public open https://example.com
+
+# Commands are isolated by session
+agent-browser --session auth fill @e1 "user@example.com"
+agent-browser --session public get text body
+```
+
+## Session Isolation Properties
+
+Each session has independent:
+- Cookies
+- LocalStorage / SessionStorage
+- IndexedDB
+- Cache
+- Browsing history
+- Open tabs
+
+## Session State Persistence
+
+### Save Session State
+
+```bash
+# Save cookies, storage, and auth state
+agent-browser state save /path/to/auth-state.json
+```
+
+### Load Session State
+
+```bash
+# Restore saved state
+agent-browser state load /path/to/auth-state.json
+
+# Continue with authenticated session
+agent-browser open https://app.example.com/dashboard
+```
+
+### State File Contents
+
+```json
+{
+  "cookies": [...],
+  "localStorage": {...},
+  "sessionStorage": {...},
+  "origins": [...]
+}
+```
+
+## Common Patterns
+
+### Authenticated Session Reuse
+
+```bash
+#!/bin/bash
+# Save login state once, reuse many times
+
+STATE_FILE="/tmp/auth-state.json"
+
+# Check if we have saved state
+if [[ -f "$STATE_FILE" ]]; then
+    agent-browser state load "$STATE_FILE"
+    agent-browser open https://app.example.com/dashboard
+else
+    # Perform login
+    agent-browser open https://app.example.com/login
+    agent-browser snapshot -i
+    agent-browser fill @e1 "$USERNAME"
+    agent-browser fill @e2 "$PASSWORD"
+    agent-browser click @e3
+    agent-browser wait --load networkidle
+
+    # Save for future use
+    agent-browser state save "$STATE_FILE"
+fi
+```
+
+### Concurrent Scraping
+
+```bash
+#!/bin/bash
+# Scrape multiple sites concurrently
+
+# Start all sessions
+agent-browser --session site1 open https://site1.com &
+agent-browser --session site2 open https://site2.com &
+agent-browser --session site3 open https://site3.com &
+wait
+
+# Extract from each
+agent-browser --session site1 get text body > site1.txt
+agent-browser --session site2 get text body > site2.txt
+agent-browser --session site3 get text body > site3.txt
+
+# Cleanup
+agent-browser --session site1 close
+agent-browser --session site2 close
+agent-browser --session site3 close
+```
+
+### A/B Testing Sessions
+
+```bash
+# Test different user experiences
+agent-browser --session variant-a open "https://app.com?variant=a"
+agent-browser --session variant-b open "https://app.com?variant=b"
+
+# Compare
+agent-browser --session variant-a screenshot /tmp/variant-a.png
+agent-browser --session variant-b screenshot /tmp/variant-b.png
+```
+
+## Default Session
+
+When `--session` is omitted, commands use the default session:
+
+```bash
+# These use the same default session
+agent-browser open https://example.com
+agent-browser snapshot -i
+agent-browser close  # Closes default session
+```
+
+## Session Cleanup
+
+```bash
+# Close specific session
+agent-browser --session auth close
+
+# List active sessions
+agent-browser session list
+```
+
+## Best Practices
+
+### 1. Name Sessions Semantically
+
+```bash
+# GOOD: Clear purpose
+agent-browser --session github-auth open https://github.com
+agent-browser --session docs-scrape open https://docs.example.com
+
+# AVOID: Generic names
+agent-browser --session s1 open https://github.com
+```
+
+### 2. Always Clean Up
+
+```bash
+# Close sessions when done
+agent-browser --session auth close
+agent-browser --session scrape close
+```
+
+### 3. Handle State Files Securely
+
+```bash
+# Don't commit state files (contain auth tokens!)
+echo "*.auth-state.json" >> .gitignore
+
+# Delete after use
+rm /tmp/auth-state.json
+```
+
+### 4. Timeout Long Sessions
+
+```bash
+# Set timeout for automated scripts
+timeout 60 agent-browser --session long-task get text body
+```